Although Iceland has many places to bathe, the Blue Lagoon is the most famous one. It's truly one of a kind. Even though many people visit, you can feel alone in the lagoon, it's big. Try the silica mask, have a drink at the bar. If you have long hair, I recommend either you don't get it wet or you put the provided conditioner in your hair before going into the lagoon, although the silica is great for your skin, it's annoying for your hair.

If it works well with your flight, it's a great plan to visit the lagoon either straight after you arrive in Iceland or shortly before you leave as the lagoon is close to the airport.