A heavily wooded single track jaunt through the center of Austin. I said out and back but if you pay special attention you can find numerous trails which connect to the trail proper. The more time you spend on the trail the more systems you will find. Take a break at the falls for a swim but be careful not to inhale.
There are several entrances. One is in the parking lot at the bottom of the ToysRus steep entrance which runs paralell to Capitol of Texas Hwy. If you are heading north on MoPac from 290 west you will take a right on to Capitol of Texas Hwy, cross over Barton Creek, and then take a left at the first light in to the aforementioned parking lot. The other entrance is at the end of Spyglass. Continue north on MoPac past Capitol of Texas(aka loop 360, BenWhite Blvd) to the Spyglass exit. At the dead end is the entrance. This is the "hill of life" a fun start but s***s at the end of the day.
http://www.singletracks.com/item.php?c=1&i=1328