Dip in Geothermal bath
No, this is not the movie with Brooke Shields. It's Iceland's most famous attraction. The Blue Lagoon is a man-made geothermal spa with hot water (app. 38C/101F) rich in minerals like silica and sulfur. The big and beautiful "jacuzzi" is fed by the water output of the nearby geothermal plant and is renewed every two days. The warm waters are rich in minerals like silica and sulfur and bathing in the Blue Lagoon has the reputation of helping people suffering from skin diseases, and even if it doesn't, it just feels great.
There are numerous tours that get you to the lagoon, or you can rent a car. It is located between Reykjavik and the airport. IcelandAir also offers transfer to the Blue Lagoon and then to your hotel for $89 (includes the admission fee). This is perfect if you are using IcelandAir's offer for a free stopover in Iceland for up to seven nights. When choosing a hotel in Reykjavik, keep in mind you will come from the lagoon with wet swimsuits, so dryer would be great.
Tickets are from $47-$234 per person, depending on what you want to have included. For us the best deal was the $65 Comfort ticket that included the entry, towel, cosmetic sample, and a free drink. Make a reservation and get there early, otherwise it gets really crowded. Even with being one of the first people at the Blue Lagoon when they opened at 8am, we still had to wait to get in because we did not make a reservation ahead of time. When we were leaving around 11am, the line to get in was really long, even for those with reservations. People say it gets emptier in the evening.
You will get a locker code and can leave all your things there. Everything is well organized, and very clean. Several hours in the Blue Lagoon is enough, after few hours you don't know what else to do. The funniest part was watching shy American teenage girls attempt to put their bathing suits on without revealing any body parts, while next to them the older European ladies are walking around naked, not worrying about their bodies at all. Very liberating.