This place needs little introduction and has become one of the most popular attractions in Iceland. It's also located close to the airport so you can also visit during short stops.
Everything you might need is there: Towels, bathing suits, ropes, slippers etc. There's also a very nice restaurant on-site.
BE SURE TO MAKE RESERVATIONS before going.
NOTE: The tours in the link include transfer and/or are part of bigger tours.