The Blue Lagoon - Bláa Lónið is a geothermal spa on Reykjanes peninsula, near Keflavik. We went there on our last evening in Iceland. This may be the most popular tourist attraction in Iceland, and for good reason: it is awesome. A giant hot tub with a steam cloud hovering over it, saunas, a waterfall and massage therapists. When we were there, it was a bit crowded but still very relaxing.
Sorry - I took no pictures.