Sleeping options when under 18 and travelling solo

If you're underage, confirm in advance with the locations you'll be traveling. Couch surfing is out of the picture if you're using couchsurfing.com, since being under 18 violates the terms of service.

Some hostels will allow you to stay if you're above the age of 16, but they usually require parental authorization. Some just have a form, some require a phone authorization, and some require a notarized document confirming consent. Having a solid plan before you go may seem boring, but giving your parents some idea where you'll be will help them sleep better at night.

Just a side note, but make sure you have access to money when you're traveling. A joint credit card with your parents, or bank account to provide quick emergency cash would be at the top of my list if I had a kid that was going to travel. You might be great at budgeting, but things happen, and being stuck in a foreign place without money sounds like a nightmare.

It might only solve a part of your problem, but there are plenty of camping sites in Sweden and Norway, where you have at least access to usually very decent sanitary installations (toilets and showers). Many of the camping sites have recreational rooms and/or communal kitchens, where you can spend some time inside, meet people and cook some food. Most camping sites also have cabins for rent. It is of course more expensive than couch surfing. Expect to pay some 15-20€/night to pitch a tent or at least 40-50€/night for a cabin. There is usually no age limit on camping sites, but at least in Norway, some sites operate with relatively high age limits to prevent youth group parties to be arranged there.

There are some restrictions near built-up areas, in nature conservation areas or in national parks, but you are otherwise in principle allowed to pitch your tent everywhere in Sweden and Norway (everyman's right).

Another possibility for accommodation is youth hostelling. Youth hostels are per-country networks of (typically) non-profit hostels (eg Scotland's and Norway's), federated into an international network, and are intended to support precisely this mode of adventure on the part of young people. They're very much distinct from the occasionally rather seedy ‘backpackers’ hostels’ that seem to have sprung up in lots of places.

Youth hostels are located in both cities and the countryside.

My personal experience of hostels is a little out of date. A friend and I went hostelling by bike in the Low Countries (from the UK) when we were 14, and I used them to criss-cross Western Europe by myself from age 16 to early 20s; that didn't seem exceptional then. That was about three decades ago (!) but I doubt the basic ethos and atmosphere would have much changed.

I can't speak to the hitchhiking aspect (never fancied that).

Please pay no attention to all the "you're only 16" crap. Just do it. Scandinavia is very safe, especially if you're blond. check if the driver isn't drunk, which is a pretty rare occurrence there, since alcohol is expensive.

There is a law in Norway the Outdoor Recreation Act of 1957, where you can pitch a tent on a field, for two days for free. http://www.switchbacktravel.com/norway/public-access

I guess this works in Sweden https://naturetravels.wordpress.com/2008/02/08/wild-camping-in-sweden-and-the-right-of-public-access/ and Denmark too.

However....If you're white, and claim you're from Western Europe, you're save from racists, and Norway has plenty of those. I wouldn't recommend this if you are a visible minority, you could be harassed, beaten up or worse.

your biggest problem is then your stay in Poland and getting to Scandinavia.

There is another community besides 'couchsurfing.com' for the exact same thing, hospitalityclub.org, where you can register and offer/ask for place to stay.

There are also a lot of groups on FaceBook for the same purpose; as well as about hitchickers and car pooling..

Bring a tent, and stay at camp sites or on uncultivated land. Bring warms clothes (wool) and rainwear.

For camping on camp sites, expect to pay ~ NOK 200 (€21)/night for tenting (example), which would usually include access to shower, WC, and a self-service kitchen.

There are plenty of cabins available. Prices and standards vary greatly, and in tourist season (July-August) can be very high.

Also, the Law on Outdoors recreation, generally known as Allemannsretten (Freedom to roam) (in Norwegian) gives you the right to pitch your tent on uncultivated land (e.g. not farm land, parks or gardens), private or public, for free, provided you

a) don't stay more than 2 nights (before moving your tent somewhere else), b) don't pitch your tent closer than 150 meters from houses, and c) are not a burden to the public.

Generally speaking, stay out of the way, don't leave garbage of any kind, and observe "no camping" signs. Make sure you leave the site as you found it. The English Wikipedia article on the subject is recommended reading.

While this doesn't answer your specific question since you're obviously trying to save money, an easy solution to your problem is to look for hotels with fully automatic check-in. Here's an example of a random hotel in Norway that has such a system:

Eight months after we wrote about a keyless test, the Nordic Choice Hotel chain has become the first to implement a 100% automated check in and departure system at a major hotel. The Comfort Xpress Hotel in Oslo now allows guests to reserve, check in and check out without ever having to deal with a pesky human.

This means that while the hotel might officially forbid under-18s from staying there, they have absolutely no way of enforcing that rule. Of course it also means you'll have to pay at least 50 euros per night, as well as having to stay in more expensive locales.