Salt lick bbq sauce. They sell it at the airport and at some grocery stores. I'll try to think of other things, but that's the one thing I had my mom ship me every christmas and birthday when I lived out of state.
Rudy's is far better than Salt Lick. Ironworks is better. That little place by the railroad tracks in Kyle or Buda (I can never remember which) is better. Bubbas in San Marcos is better. Lamberts downtown is better. Rubys is better. The BBQ at my old tailgate is definitely better. ETA: County Line is better.
Rudy's! Yum! In Round Rock they have these giant donuts. They're reallyyyy good and they were on one of those food network shows. Austin is kind of one of those places that you go for the experience. They have a big music and art scene. I'm personally not a huge fan of Austin but it was the nearest civilization to Ft Hood so you take what you can get.