When you land, pick up the rental car and head immediately south on 183 for 25 miles to Lockhart, home of the best BBQ in the world.
Stick to small to reasonable servings, since you have at least 3 different restaurants to visit for lunch: Blacks, Smittys, Kreuz. Lots more info on the chowhound.com TX board.
http://www.texasbbqtrail.com/lockhart.php
After lunch, I second the suggestion of Barton Springs. Austin in August will be HOT, and the springs will be great.
I'd suggest skipping 6th street in favor of some live music. Not that 6th st isn't cool, but Austin calls itself the live music capital of the world, so you gotta do some, right?