There are few geothermal pool options when traveling on the Golden Circle. Fontana spa is definetly recommended. This spa is one of the most family friendly spas you'll find apart from the local swimming pools.

Laugarvatn Fontana consists of two pools, a hot tub with a great view of the lake, steam room, sauna, and the lake! Jump into the lake in between dips in the hot tub for great lymphatic drainage.

Children 12 and under are free. Towels and swimsuits are available for rent.

Also consider doing the Icelandic rye bread tour which starts here. The short tour shows you how to bake bread in the ground using geothermal heat and you'll get a taste of the bread too!