Where can I go to avoid planes overhead?

This web site shows the location of all (registered) flights that are currently in the air https://www.flightradar24.com/

You can check for a specific location or just look at the overall pattern. Obviously there are a lot more flights in the northern half of the globe and flight density is a lot sparser the further south you go.

At the time of this writing, there are no planes across the Amazonian area and large parts of Southern South America (with the exception of the Pacific coast). There is only one flight in the entire South Atlantic south of St. Helena. Tibet is mostly flight free. My current location is free for about 10 miles but BA175 from LHR to JFK will pass over in about a minute or two.

Donbass

This is a region in eastern Ukraine under control of pro-Russian rebels. Airlines used to freely overfly war zones with low level conflicts like this, because the belief was rebels only had small shoulder-fired SAMs like Stinger and SA-7, which were short-range. Actually, the rebels had Buk and other advanced, truck-mounted high altitude SAMs. They were vigorously targeting Ukranian Air Force planes (who had started flying high to avoid SA-7s), until they accidentally shot down a Boeing 777 jetliner, Malaysian MH17. Now airlines avoid it like the plague.

The risk may not be high now four years gone, but the basic political situation on the ground has not changed. The same rebels control the area, and they still have high altitude SAMs. So if an airline routed a flight over it, and somehow their airplane did get shot down, they could not possibly justify it: their civil liability would be out of this world. The dispatchers might even go to jail!

The Ukranian military doesn't like to fly there, same reason. The Russians don't like to fly there because they don't claim it's part of Russia, and they don't want to stir the political pot by letting Ukraine and NATO catch them violating the Ukraine border.

Also, here's an interesting (but by no means conclusive) map.

The middle of the Tibetan plateau might be an option. Long-haul airliners don't fly there because in case of a pressurization failure it would take too long to reach air with safely breathable pressure.

It ought to be possible to find a place that avoids the few domestic routes to/from Tibet's own airports that are pointed to in comments -- especially in the western part of the plateau.

Antarctica should be a pretty safe place; the only flights you would need to worry about are the flights to and from the bases which will be few and far between (especially when compared to the daily flight activity over pretty much every other continent) and a very limited number of tourist flights. Part of the reason is that there are no scheduled flights from either of the three southern hemisphere continents that pass over Antarctica proper. In case the evidence there is not enough there is also a non-SE source which points out why they don’t do it and why they would be unlikely to detour even with (un)favourable winds.

You can improve your position in Antarctica by staying away from the flight routes from the bases to their restock positions (i.e. the direct route to Christchurch for most of them). I couldn’t find a map of where and how far south the tourist flights go (they don’t land, they attempt to show a scenery) but I suspect they wouldn’t go too far inland.

You can go to the vicinity of a volcanic eruption.

https://www.businessinsider.com/why-planes-do-not-fly-during-volcanic-eruption-2017-11

I do not know how you will get there. You can not fly there.

Last year I spent a week's holiday on the western-most point of the island of La Palma, one of the Canary Islands. Had a great view of the Atlantic Ocean and didn't see any aircraft all week long.

I am surprised that no one mentioned Beijing. Downtown Beijing, within the 3rd Belt Highway, is a no-fly zone except for the occasional military aviation. Also, the smog makes it difficult to see anything in the sky :-) Good chances are that, in a month's time, you won't see any aviation at all.

It seems to me that the city of Paris is also a (sort of) no-fly zone, in that aircraft may not fly lower than 6,000 ft. Both Paris and Beijing's airports are way outside of the central city. However, Paris is very small, so I'm not sure if this works.

I found an interesting visualization called Flight Stream that connects the worlds major airports with each other and simulates air traffic. While the flight paths are not accurate, this does show the type of patterns you would expect in a 30 day period, and the outlook isn't good for your question. After looking around for a while, I can't find a place other than maybe parts of Greenland or extreme North Russia. The North Pole isn't technically land, but it seems pretty free of flights according to this.

Contrails from large jets can be seen from 200 miles away, so when you look at this map, keep in mind that each of the flight paths should be about 200 miles wide. You're basically looking for a 400 mile wide area that has no flight paths over it (something like the size of Wyoming).

Again, this is not an accurate representation of actual air travel - it's more like a worst case scenario of where planes could be.

Experiment to map many of the airline flights between world airports. It's not showing real time positions (which would be amazing but I don't have that data) but rather, great-circle routes between major airports based on flight data from the Open Flights site. As a visualization, I think it fails since there is so much data that around major airports, it just blurs into a mess. Still, it was fun to do and looks kind of pretty (for some definition of pretty).

Anywhere that's not too close to an airport and cloudy with low ceilings would work, as most planes would then be flying above the clouds, where you can't see them, and too high to hear.

The simplest option is just to stay indoors (and not look out of the windows). To do that for a long period of time, I’d suggest a northern city that has a very cold climate in winter, and is set up so that you can get around in the city without going outdoors — Montreal, for example.

I lived in rural Nebraska along the Kansas state line and only occasionally saw a commercial jet. In 5 years I never actually heard one -- just barely saw one way up high.

What I DID see, though, was crop dusters. I recall waking up at 7 am on a Saturday morning as they were buzzing the house to spray the field just outside of town.

Antarctica?

As is frequently shown in debates between flat Earthers and sane people, there is no commercially viable route that actually crosses Antarctica on a great circle. South America, Australia and Africa are spaced at roughly 120 degree intervals around the South Pole. (There's one route that occasionally clips an edge of the continent if wind causes deviation from a great circle route -- I have forgotten the details).

Your choice might widen if you are willing to disregard airliners at cruising altitude, which are totally inaudible and all but invisible except when weather conditions favour the creation of contrails.

If you want somewhere more habitable, Tristan da Cuhna appears to be in almost the same category as Antarctica. The only possible route I can see that more or less crosses it, is Sao Paolo to Cape Town. I don't know if that route exists.

As a general answer - remote locations in narrow deep valleys are potential candidates. Factors that influence suitability include proximity to 'civilisation', major cities, airports of any sort and air routes. I live in New Zealand. Much of the southern part of our country consists of extremely rugged and mountainous terrain and I am certain that numerous areas would meet your requirement - but it would take research to establish which are most liable to.

Once you found a NZ area that met your specification apart from random unfortunate incidents it may even be possible to obtain the cooperation of local aircraft operators to stay out of a very tightly defined area. You'd no doubt need to satisfy those concerned that you did not have ulterior motives.

You specifically mentioned chemtrails. You may have to take special security precautions to hide the plan from the chemtrail establishment and it seems logical that anyone in the chemtrail generation business would want to crash any ;party' that seeks to exclude them. This applies for any site you choose.

The Chatham Islands are almost 1000 km to the East of the New Zealand mainland. There is an airport used by flights to/from the mainland (located on the tongue of land between the upper and lower lagoons approximately central to the island. There will be locations on the island where the topography and location would make mainland - Chathams flights inevident.

The extremely remote and bleak "Auckland Islands" are located almost 1000 km to the south of the NZ mainland. There is no airport. Helicopters occasionally visit - almost exclusively on rescue missions when scientific parties 'get into trouble' - as happens occasionally - when the helicopter flights are rare enough to be news worthy. (One was lost near the islands recently and the crew of 3 were extremely lucky (as well as skillful) to survive).

If more ruggedness and isolation is required the further still to the South-East and unplesantly closer to Antarctica Campbell Island would almost certainly 'fill the bill".

An option might be North Korea. For political reasons international flights go around instead of over, so there would be much fewer overflights than in most other inhabited places.

Most scheduled international flights from Pyongyang go west across the Yellow Sea, but Wikipedia says there is a route to Vladivostok. ATC maps suggest it flies due east across the country and then up the east coast.

There are also some domestic flights, and I'd imagine information about their exact flight paths is very hard to come by. Finding a place away from the domestic routes will to some extent be a matter of guesswork; the best you can do is probably to pick a place that is not near the straight line between Pyongyang and any of the larger cities. Somewhere like Hwapyong looks like a relatively safe guess.

Getting permission to go there will no doubt be an adventure in itself.

There are way too many places that will never see planes overhead; and it is very hard to prove a negative.

Northern Canada, Alaska, Northern Europe, Probably most of eastern Russia, Center of Africa, as well as the center of the Amazon and southern Argentina.

and most probably in the middle of the South Pacific ocean.

This might help. Of the options presented, I think the Taj Mahal would be a good choice. Or the Boundary Waters Canoe Wilderness Area might be what you're looking for:

https://en.wikipedia.org/wiki/Prohibited_airspace

The Boundary Waters Canoe Area wilderness in Minnesota and Ontario is not, as I had thought before researching this answer, off limits to planes. There is a minimum altitude for planes of ~850m (2,800 feet), which would mean that you could possibly see and hear them, but I have never seen or heard a plane or contrail there.

But then, I haven't spent a whole month up there. 14 day limit at one campsite, so you'll be moving, and you have to pack everything in and out.

Northwest Scotland (except military jets)

There aren't any major airports in the northwest of Scotland - your closest places that take jets are Glasgow (south), Edinburgh (southeast) or Aberdeen (northeast), and international flights don't tend to go that way.

On the downside, Scotland is (or used to be) a major training area for the military. In the 1980s when I went there as a kid, most days you'd see either F111s or Tornados, depending on who was practising. I don't believe it's as active these days though, because most of the old Cold War squadrons have either been disbanded or have been deployed round the world. So you're reasonably likely to be plane-free.

You won't hear or see trails from planes in pretty much 80% of Australia. Of course you won't see much else there either.

You could definitely get out a lens and see some planes maybe 2-5 times a day, but you'd be hard pressed to actually visually identify any planes.

The Australian climate don't allow for planes to have plane trails most of the time.

From the SkyStream page linked in JPhi1618's answer, there appear to be a couple of dry-land options in the South Atlantic & South Indian Ocean that might work.

• South Georgia Island, a UK possession, has permanent structures and a small population of scientists from the British Antarctic Survey plus a few government officials. Its location in the South Atlantic lies on the great-circle route between Tierra del Fuego and Cape Horn, which means that not a lot of commercial flights would be expected to ever overfly it. Antarctic cruise ships and yachts chartered from the Falkland Islands do regularly visit, so there's actually a reasonable method to get there & back. Accommodations might be an issue, though.

• The Prince Edward Islands are South African possessions in the southwestern Indian Ocean. They are officially a nature preserve, with a few dozen of scientists living on them at any one time, but no permanent population. The island had a problem with feral cats hunting the seabirds, so they eradicated the cats, and now the island is overrun with mice — which are also attacking the seabirds. Maybe you could volunteer to help out with that while you're there.

• Similarly, the Crozet Islands are French possessions in the southwestern Indian Ocean. The situation is similar to the other two: lots of seabirds and a few scientists. These islands, however, are starting to get a bit close to the Johannesburg–Sydney great circle route, so there's a greater risk of seeing a plane here than if you visited one of the other two.

Ask the US government if you can come visit Area 51 in Nevada, planes are not allowed close.
And while you are there, take some pictures of the aliens too :-)

You can achieve what you want on any latitude and longitude - even in on the coordinates of busy places such as New York, L.A., Berlin etc. - as long as you are willing to climb to a very high altitude.

During the period from 2001 to 2009, 7 space tourists made 8 space flights aboard a Russian Soyuz spacecraft brokered by Space Adventures to the International Space Station (...) On June 7, 2019, NASA announced that starting in 2020, the organization aims to start allowing private astronauts to go on the International Space Station, with the use of SpaceX's Crew Dragon spacecraft and Boeing's Starliner spacecraft for public astronauts, which is planned to be priced at 35,000 USD per day for one astronaut.

Planes only go higher than the International Space Station on rare occasions. For example, the Hubble telescope might need servicing, so a Space Shuttle (a special kind of plane) would be sent with the crew to fix it, and the Hubble does fly 160 km higher (~100 miles) than the ISS - but it very rarely passes over the ISS, and the Space Shuttle is now retired. Seems like there isn't a replacement for it yet and there won't be one for the next year, so it'll be prime time for you to do some astronauting!