The Greenbelt is one of the best things about Austin. It's foresty and the water's warm enough for swimming. The trees and brush open up for little coves that offer a lot of privacy and give you the illusion that you are alone in a little lagoon even though everyone else in Austin is swimming nearby. We went during spring break, and even though it was packed, we were able to find a swimmy little spot away by ourselves.
If you have an overactive imagination you might feel like Huck Finn exploring the set from The Blue Lagoon. There's a rope swing, people!
It's a beautiful place to hike and take a dog, even those that can behave unleashed.