After the Sri Sangwan waterfall, we went back down the road a couple miles to the Pong Arng hot springs. They were a pair of natural hot water springs coming from underground. The water was scalding hot and smelled like rotten eggs. The flow hot water was combined with a flow of cold water and the result was diverted into a couple pools where people can get in and soak.
But if you're thinking about going to the springs, realize there are rules to obey. My favorite? Don't be a loud nois (sic)!