It is of course must to visit the Blue Lagoon if you haven't already. It's around 40 min drive from Reykjavik but I recommend that you either visit it on your way to Reykjavik or when you are going back since it´s on the Reykjanes Peninsula like the airport and can be ideal specially if you are arriving early or departing late.
The Blue Lagoon is one of Iceland's most popular attractions, and it's is little wonder as to why. The beautiful milky-blue water is unlike anything else found on earth, and makes a stark contrast to the surrounding black lava field and its creeping grey moss. It sits at 39°C (102°F) year-round, making it the perfect temperature for bathing.
The lagoon is situated on the Reykjanes Peninsula in southwest Iceland, a region renowned for its barren landscapes and cone-shaped volcanoes. It is only a 15-minute drive from the Keflavík International Airport, and around a 30-minute drive from the capital city of Iceland, Reykjavík. That makes it the perfect first stop when arriving in Iceland, or the last stop before you leave the country.
Due to its excellent location, its soothing waters and rich skin nourishment, the Blue Lagoon quickly became Iceland's most visited attraction, rivaled only by the Golden Circle sightseeing tour.