I find all cities boring and skippable. I would rather go to the outskirts of Houston to walk on the USS Texas, or Corpus Christi to walk the USS Lexington, than set foot in Austin or San Antonio again.
Regardless, here are a few more thoughts on the Austin area:
The LBJ presidential library is in Austin, if you want a historical thing. It was alright. It has a decent museum and interesting architecture. If you know about LBJ, there isn't much to learn in the museum, but it does have "artifacts" to look at.
Lake Travis is the best thing in the Austin area, but I'm not sure the best way to see it. Rent a boat, I guess.
Touristy Austin is a food, alcohol, and live music. You could always just stop for lunch at one of the good Texas BBQ restaurants downtown. It wouldn't take too long and you kinda get to see where some of the Austin music scene happens. I think
Furry
knows the good places downtown.
If you have a few people to drink with and want to make a day of it, you could get up real early and go float the Guadalupe river in New Braunfels or go to the highly rated water park there, Schlitterbahn. After a day of floating in the sun, it's only about 30 min or so to downtown San Antonio. It's not my thing, but I know tons of people love it.