|Magnificent rocks in front of the campsite|
If you plan on doing this activity, I warn you now, that you cannot be the timid type. There may or may not be nudity.
The first one we went to was in Montana, called Lolo Hot Springs, it's adjacent to a campsite. It hasn't been upgraded in a few years, so it's quite old school. They have an outdoor and indoor pool, a restaurant, a bar and snowmobiling in winter.
Second one is in Idaho (a short drive from Lolo) called the Jerry Johnson Hot Spring. It's free but there is a short and easy hike to get there. There are two springs, they're a bit muddy when somebody gets up or gets in the pools, but otherwise, its a nice soak.
The third one is my favourite, it's also in Idaho, Weir Hot Spring. This place is one of those places that you have to be in the know to find, it is unmarked outside. The trek is NOT easy - we even had to put our feet in very cold water to cross in a couple of the areas, but... so worth it. It was lucky that when we finally got to the spring, there was nobody else there, it added to the feeling of finding your personal secret paradise.