Harbin and Sierra Hot Springs in California
I've visited Harbin Hot Springs, north of the San Francisco Bay area, and recommend it. You do have to pay to get in, because it's not so much a recreation area as a holistic health and meditation center, oriented to improving your relationship to your body and your soul. They have a variety of pools of different temperatures, and some are silent pools. No radios or music players are allowed. Clothing is optional throughout the center, even in the pools, but nudity is the norm except where you can be seen from the main public road.
https://harbin.org/
There's also Sierra Hot Springs, which I'll be visiting later this week. In this one, clothing is mandatory everywhere but the pools, which are clothing-optional. It's run by the same institution that runs Harbin Hot Springs.
https://sierrahotsprings.org/
Made it to Sierra last weekend. Very different vibe from Harbin. As for the springs, I found them a bit too sulfurous for my taste, but that seems to be a condition with many of the springs in the Sierras, due to geothermic volcanic activity. The sulfur was about 45-50 mg/liter. The main swimming pool was better. Like Harbin, cameras and electronic noisemakers are banned, and silence is encouraged in the pool areas.
I have been a life member of Harbin for the past 15 years. I enjoy the quiet solitude for self meditation in the pools surrounded by hills and newly sprouting forest after the 2015 Valley fire that destroyed most of the old buildings. They have done a wonderful job rebuilding the pools and sauna facilities along with replanting native forests which is re-growing and thriving wonderfully.