A few things I'd pass along.
Plan on a minimum of 3-4 days.
Austin is a fun town. If you are really into live music stay there. If not stay in San Antonio or New Braunfels. Especially if you have kids or not into that scene.
San Antonio offers more touristy things and should have much cheaper lodging.
BBQ: Don't be an idiot and stand in line for hours at Franklin BBQ. Drive to Luling, Lockhart, New Braunfels or Driftwood to get awesome BBQ. You'll actually get a table and enjoy it more. A few names to research for BBQ: Salt Lick, Black's, Cooper's, Kruez, City Market, etc...Roadside BBQ at Rudy's is solid.
Many spots offer complimentary beans...so find out first. Cabrito at Cooper's is usually really good. Beef is King...pork is hard to find. If you like lean Brisket make sure you request it.
Schlitterbahn in New Braunfels is awesome. Best natural water park in America.
The Paseo Del Rio (River Walk) is really fun for drinking. Take the barge ride to view the River Walk early in the morning. If you wanna save a few bucks stay 1 or 2 blocks instead of overlooking the river.
Fiesta Texas is fun for the kids but pretty much similar to other theme parks.
Air Travel: SAT or AUS. Non-stop flights from TPA, PIE, MCO and SFB
Weather:. Expect 95-100 degrees. USF section will be on the sunny side.
Alamo: Do it only, you need an hour. Literally about 300 feet from the Riverwalk. If you have a day trip planned. Try Fredericksburg, Boerne, Bandera, Wimberley, San Antonio Missions
Parking on gameday: I used to find parking south of campus on the north side of downtown. Have not been to a game at UT in 10 years though. If you are in good shape the 3/4 mile walk to the stadium might be the way to go.
Finally for you Football fanatics: SMU is at Texas State on the same day. Texas St is about 30 mins south of Austin on I-35 heading south towards San Antonio.