Stay downtown when in Reykjavík – everything is walking distance and the shuttle pickups for tours don’t take long.
Self-cater or eat your main meals at lunchtimes. Food prices are extortionate.
Get away from the city & and light pollution on a clear night if you want to see the Northern Lights. (Check out the official forecast a few days beforehand to plan this.)
Practice with the manual settings your camera before you arrive to get photos of the Northern Lights. You’ll need to be able to change the shutter speed on your camera.
Hire a car and head to the north and east coasts if you want to escape other tourists.
Go soon. Tourism is increasing massively and changing the country (300k population, expected 1.7m tourists in 2016 with increases by a third every year). In ten years’ time it will be very different.
Eat puffin. It’s endangered as of last year due to the increased demand from tourists who want to try the “local cuisine” (except locals don’t actually eat it). Similarly with minke whale – whaling has been illegal for decades. If you really want to try weird food that locals actually eat, go for foal or reindeer.
Walk on the moss – it’s very fragile and takes up to 50 years to recover from a heavy footstep. Don’t be Justin Bieber.
Use your flash while taking photos of the Northern Lights. It will do absolutely nothing for your photo and ruin everyone else’s around you.
Go to Blue Lagoon if you want the real deal. It’s nice enough, and admittedly some people love, but it’s also pricey and you won’t find any Icelanders there. For the real experience go to a local pool (be prepared for showering naked before you’re allowed in) or book on a tour that will take you to a natural pool as part of a hike.