Salt Lick, Franklin, and Stubb's are all amazing for BBQ. Eat at least one of these places to get a genuine taste of Texas-style BBQ, bonus points if you eat at all three.
As far as 6th street goes, it's not the live music extravaganza it used to be, but there are still some good acts out there. I've noticed 6th street's attitude depends largely on what's going on with the college at the time. I imagine UT is done for the school year, so there will be less frat boys and drunk teenagers on 6th street. I imagine it will be more relaxed than usual (but still pretty wild on Fri-Sat night).
Check out Waterloo Records, the coolest music shop in town.