South Congress is pretty cool area to visit. 6th St. if you like to bar hop. Home Slice Pizza and Guero's are both good places to grub. Torchy's Tacos were pretty good last time I went (lime pork tacos were the best, IMO).
-- There is a little operation called Blue Starlight Mini Drive In Theatre that is kinda cool. It's in a parking lot, but still cool. Alamo drafthouse, of course.
-- There is a bar called Brew Street that sells beer based on demand. Nice idea, but that building has the most shitty signage ever. It's literally covered in rust and has no real sign to speak of.
--If you have time, drive out to Round Rock and get some Round Rock doughnuts. It was on Man Vs. Food, oh so tasty. MvF also visited Juan In A Million, although I wasn't overly impressed with that place.
Here is a collection of things to do: http://www.austin360.com/this-is-austin/
I don't live in Austin, but like to visit every couple months. Austin is the city that Nashville wants to be.