Spencer's hot spring just east of Austin is my favorite spring in the state. Its pretty easy access in the winter from hwy 50, its about 2 miles from the highway on a dirt road. There are also some other interesting things if you're interested in petroglyphs or pictographs and/or fossils. But they can be harder to reach. Camping is free and completely undeveloped. My favorite pool there is one of the cattle troughs, but there is a nice rock pool too (can be pretty hot sometimes). It used to be total seclusion out there but more and more people are finding out about it. Ask away if you have any questions.