Icelandair hótel Reykjavík Natura National Museum is amazing if you're into that sort of thing. Climb to the top of Hallgrimskirkja. Rent a car for the golden circle trip and to explore the ring road. Plan for a lot of time at Thingvellir--it can definitely be half your day. Make sure you visit the Blue Lagoon. Do a whale-watching cruise. Also you owe yourself a trip to the phallological museum because where else can you actually see a bag of dicks?