Coming out of hiding here...
Reimer's Ranch, about 40 minutes from Austin is a great place to go mtb. I just went last week and had a great time! A shadier, closer option in North Austin is Walnut Creek. Less varied and less trail, but still fun.
If you do ride the Hike and Bike you will likely be dodging lots of pedestrians, stroller pushers, dog walkers and runners. Early morning weekdays are the best times to avoid the crowds. At the west end of the Hike and Bike is the Barton Creek Greenbelt, which is fun on a mtb.
The Chronicle is a great resource for finding music and other fun stuff! I can ride some weekdays, possibly on the weekend, too. I do both road and mtb, and do like to take my fixed gear out on the Hike and Bike way out east now and then. I live in South Austin, and the Hike and Bike and Greenbelt are just a couple of miles from my house, as several good road routes.
The bicycle is the most civilized conveyance known to man. Other forms of transport grow daily more nightmarish. Only the bicycle remains pure in heart. ~Iris Murdoch, The Red and the Green