Voodoo Doughnut is a must-stop.
And for a hole-in-the-wall, one-of-a-kind sort of retro experience, check out the Austin Toy Museum. It is NOT a "museum", but rather an odd conglomeration of sometimes poorly and other times cleverly displayed toys and action figures from the '80s, mostly. Admission gets you free drinks and access to the arcade, as well, which is really the bigger draw, IMO. If you or anyone in your party appreciates vintage arcade games, pinball and even television console games, pay a visit. There's no time limit, and if you go in the afternoon, there's no crowd.