You’ve probably already driven through, but for the drive back there are two good barbecue places in Austin just a block west of 35, Stubbs at Red River and 8th, and Iron Works a little farther south at Red River and Cesar Chavez.
ETA: Sorry, just saw you’re trying to avoid bbq. 
If you’re wanting burgers, Hut’s Hamburgers is about a mile west of 35 on 6th street and is one of my favorites.
If you’re not sure what you want to eat go West on 6th St and stop and Whole Foods. Eating in a grocery store doesn’t sound very exciting, but trust me, this one’s a bit different.
I ultimately didn’t have time to stop for a real lunch, so I grabbed a sandwich during a stop for fuel. I decided to stay the night in Austin on my way back, so I had dinner at Baby A’s. Eh. It was okay, but I’m sure I could have done better if I’d known my way around.
The next morning I met a friend for breakfast at Taco Deli on Lamar and then had some coffee at Houndstooth before getting on the road back to Dallas. Thanks for the suggestions, everyone!
I was going to recommend Torchy’s Tacos or some of the gajillion food trucks, but then I see read your name, so maybe not so much. Hope your audition went well.
BTW, people, I fricking love Torchy’s Tacos–that’s where I always want to go when I visit my fam in Austin!
We have Torchy’s here in Dallas, but I would have been interested in sampling some food-truck cuisine if I’d known where to go. Maybe next time.
Baby A’s, seriously, people? It’s fine for margaritas but doesn’t even rate for Tex Mex. Hell, Chuy’s is better.
Gray Ghost’s rec of Franklin BBQ is probably the best “near the highway” recommendation. That whole East 11th corridor is getting a lot of love, with the food trailers there (try Tony’s Caribbean Grille). Three Little Pigs is a trailer that might be the next big thing - pork a number of different ways. (Sorry Troy, it’s Texas. You’re not going to get a lot of healthy specialty options right off the highway!)
My wife has eaten at the Whip-In and she thought it was pretty good. It’s definitely got the kitsch thing going, very Austin and hip.
Paul Qui’s East Side King on E 6th Street. It’s a food truck/stand