If you want to have an icelanding geothermal pool just for yourself, then Strútslaug is the way to go. Completely lost in the middle of Central highlands 2 steps form glacier, here is wildernes.
I do not put a lot of personal opinions here on this website, but to be honest, I personally didn´t like Strútslaug so much. The water was really hot and somehow bleary.
How to Find Strútslaug:
Here is the place where you can park your car (GPS: 63.843009, -18.955789). From here on, you have to rely for good 5 kilometers over the hills and valleys on your own legs. Good travel to you, explorer!
GPS coordinates of Strútslaug geothermal pool: 63.874937, -18.944599 Take me there!
Photos of the Strútslaug natural geothermal pool:
The Nearest Natural Geothermal pools