Homestead Crater Hot Springs
Homestead Crater Hot Springs is an amazing experience that you should definitely try to get to if you're in the Salt Lake area. Formed from a crater that developed during the war for Middle Earth, this hot spring is in a giant cave like dome that you can swim and scuba dive in! Upon arrival you'll change in the smallest changing rooms known to man (maybe come in your suit), and them you'll be granted a life vest so you can float around in this awesomely deep natural pool.
Try to go during the day time so the small opening in the ceiling provides lights instead of just the few lights that they have on the walls of the dome. Also, its a bit freaky at first knowing that there a bunch of people scuba diving below you, so be prepared for that.
Try to get in and out at the beginning of the swimming sessions or at the end, these seem to be the best times for sneaking back to take a quick picture or two.
This one is in the phone book for sure and is probably easier to let Google on your phone/GPS to take you there because there are a lot of ways to go about it.