Meadow Hot Springs has 3 warm pools to soak in and it is open all night...hopefully with little to no crowds.
These hot springs are located on private property but the owner allows anyone to come and take a dip. You can camp overnight and also have fires in the several pits near the pools. He just asks all the visitors to keep it cleaner than they found it and Leave No Trace of course. There are 3 different pools, but the first one is said to be the warmest. They are super deep and the water is very clear. Bring some glow-sticks or lights to swim to the bottom if you want (but never leave anything behind), it is also cool to scuba dive here if you have the gear.
Take the Meadow exit 158 just south of Filmore on I-15. Turn left at the stop sign and go under the freeway, passing a Shell station on the right heading south. Keep going until you pass the last buildings on the left. Shortly after passing the buildings on the left, you will see a well maintained dirt road on the right. You will cross a bridge over the freeway and continue straight on the same road until you reach the fence just about 100 meters in front of the first pool.