Hot Springs State Park in Thermopolis (aptly named), WY. The land was deeded to the state by a local Indian tribe with the understanding that the springs would be open for free to the public. You get a 1/2 hour free in the hot-spring-fed pool, but can't actually get in the springs as they are too hot. You start in the warm end of the pool and then get as close to the incoming hot water as you can stand. The park was beautiful with fantastic formations from the mineral-rich water building up.
Also, the abandoned hot springs resort that is on the campus of the Armand Hammer United World College in Las Vegas, NM. Yes, NM, not NV. There are several small pools left behind that range from "lobster-cooking" to "bearable", but they're the "stinky" kind of hot springs, but supposedly very good for your skin. I didn't have a swimsuit with me and it was too close to the road for skinny-dipping so I had to settle for soaking my tootsies.
Breitenbush Hot Springs in Detroit, Oregon. The chance to enjoy some of the remoter springs "au naturale".....
The historic Baths in Saratoga Springs. A soak in what felt like hot murky salty fizzy gingerale followed by an hour's massage followed by being wrapped in a warm sheet and being put to bed for as long as I wanted to sleep (so many rooms available, they didn't need to kick you out for the next person). I slept for a couple of hours and then walked through the park as the snow fell softly on the pines. Sublime.
One of the historic spas (can't remember which one) in Hot Springs, Arkansas. You move from pool to pool, each one hotter. Then you reverse the process. Then you get a massage. At that point, you could die happy. I guess I wasn't quite happy enough, 'cause I'm still here. The fountains all run hot there. If nobody yells at you, you can soak your feet.
Ojo Caliente Hot Springs in New Mexico. Paradise. Several different pools with different types of springs feeding them. There was soda water, sulfur water, arsenic water, lithium water (woo-hoo), and a mud bath (fun!). One pool had a natural bottom with tiny smooth rocks; you could dig your feet into the rocks and it felt like a foot massage. There was also a huge body-temperature mineral pool. The pools were open til midnight with just enough lighting to barely see your way around and one night I was floating in the body-temp pool, looking up at the most blazing night sky I had ever seen. I felt like I didn't even have a body. And if you were ambitious enough, you could drag yourself out of a mineral pool, have a massage and stagger back to a pool. I didn't want to leave.
My sister was telling me about a beach in Baja California that has the hot springs that ooze up through the sand. People come down to the beach, dig a trough deep enough to fill up with hot salty water and soak. I'd love to do that.