Going to the Blue Lagoon is a must when in Iceland, so we rented a car and drove from Reykjavik (very easy drive). It was around £35 for admission with towel hire, but you can reduce the cost by bringing your own towel.
The day we went it was below freezing and extremely windy which slightly ruined the experience for us. The changing rooms were clean, lockers were big, and it took us about 15 minutes to build up the courage to run outside in our swim wear and jump into the lagoon! When we did eventually get in it was lovely and warm and we had a nice paddle around. Because the wind was so strong, in the areas of the lagoon without shelter there were waves and water blowing into your face (not a problem for most but I was wearing contact lenses and the water blew into my eye and clouded my lense so I had to get out to wash it). There is silica available to put on your face as well.
The worst part was getting out from the warm lagoon, soaking wet in the strong icy winds! Only after getting out did we realise that indoors there is a small children's pool, and if you get in there then there is a door that you can swim through which links to outside... I wish someone had told us that because we'd have stayed a lot longer!
All in all, quite expensive for what it is, but definitely worth a visit.