Anything to do in Reykjavik airport at 9 am during a 1h30m layover?

1:30 is not a layover. It's a mad rush between gates.

Remember, practically all flights board at least 30 minutes prior to departure, and on large aircraft it's often an hour or more ahead of departure.

This means that once you land, you'll have time to run to your departure gate in time to board, and if you're lucky, hit a restroom on the way.

Do not plan anything. If you get to your departure gate before they leave (most likely due to a delay), then look for something in that immediate vicinity to do--such as a cafe for cup of coffee, or a gift shop to browse.

An hour and a half is not time to be a tourist--unless you intend to miss your flight.

Yeah, 1.5 hour is not much time. I had a similar layover in Reykjavik to Amsterdam before too.

I killed some time by having some coffee and looking around some shops at the airport. There is a souvenir shop with interesting things like Icelandic troll figures. There is also a nice sculpture, which I saw a video of on my flight on Iceland Air. It is a just a bit annoying buying anything there because you get change in Icelandic kroners.

I'm surprised no-one's mentioned it, but Reykjavik is at least 1 hour's drive away from Keflavik airport, so I don't think sight-seeing is on the cards.

just got back last weekend from doing a similar layover in Iceland - the area where you get off the plane after customs drops you right into a crowded waiting area, so go explore the airport past the waiting area and you'll find lots of shops and a nice place to get a great coffee, Kaffitár.

I wish I changed some money because I like having a couple bills in foreign currency as souvenirs, but didn't need to by paying for everything with the card.

I think you'll find an hour and a half is barely enough time. You need 20-30 min or so getting off the plane and going through customs. Queueing for the next leg, you'll want to be boarding at the gate 40 min before takeoff. This only leaves you with a half hour for that coffee. Assuming everything is on time.

As mentioned, Reykjavik is 40 min by bus, and then only every hour. Take a full day for your stopover (especially since Iceland Air doesn't charge anything extra) next time - visit the blue lagoon and Reykjavik then - totally worth it!

An hour and a half isn't enough time to go touristing from any airport, of any size, anywhere. You need to be back at the airport for your onward flight at the absolute minimum one hour before its scheduled departure, and often an hour and a half to two hours. Add to that the time it takes to get off the plane, get through immigration, travel to the city and travel back to the city and you need a layover of multiple hours before you can consider doing anything outside the airport.