If you want to have an icelanding geothermal pool just for yourself, then Strútslaug, or soemtimes called also as Holmsarbotnalaug, 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!