Small but nice spa on Road No. 37 between Pingvellir and Geysir.
Basic entrance approx £15 as of May 2013.
Everything is outside, like often in Iceland. There is a very shallow warm pool, a small deeper pool a couple of celcius colder than the shallow pool, a big hot tub, a sauna and geothermal steam rooms.
The latter are interesting as you can hear the spring burbling & gurgling underneath you. This makes the temperature inside a bit unpredictable but it's certainly uncommon!
Note that in Spirng 2013 some construction works were going on somewhat spoling the view of the lake with ugly steel fences.
Despite this, it is a very welcome stop after a hard day of sightseeing on the Golden Circle!
Favorite thing: During our stop at the rather un-scenic gas station, I was able to take a few photos of the rather scenic surroundings. The lake and pleasant landscape beyond was quite enjoyable to look at.