There's a possibility you'll be too early for the bats. They arrive late March through mid-late April, it just depends.
However, Austin never runs out of things to do:
1) Great music, not just on 6th Street but in multiple venues around town.
2) Take a tour of the highland lakes.
3) Bluebonnets will be starting to erupt everywhere - Burnet has their bluebonnet festival (small but cute) April 9-11.
4) Educate yourself about BBQ, Texas style. Take a trip East to Taylor for Louie Mueller's, south to Lockhart for Smitty's (my fave), Kreuz's, or Black's, a bit farther south for City Market in Luling, or west to Llano for Cooper's.
5) Visit the State Capitol
6) Have a drink in the lobby bar at the Driskill
7) Visit Zilker Park and see the botanical gardens or take a dip in the Barton Springs Pool (cold even in summer)
Check
Austin 360's website to check music/entertainment in the area as well as restaurants and events.