Hamilton Pool is the most beautiful swimming hole I've come across in the Austin area. It is a county park, so admission is $8/day, but it is so worth it! The pool is surrounded by a tall natural rock wall full of romantic waterfalls. If you don't feel like swimming, there are also some beautiful hiking trails around. But go early! Admission is limited to a certain number. It makes the environment great, but if you come when the park is at capacity, they only let cars in as others exit.