Lots...
1. swimming at Barton Springs
2. biking along the Greenbelt
3. Visit the capitol
4. Take tour of UT Tower
5. The drag on Guadalupe
6. Go down to Town Lake
7. Get drunk on 6th Street/Warehouse district
8. Go to Zilker Park
9. Sailing/Boating on Lake Travis
10. The Oasis (resaurant) on Lake Travis at sunset (highly recommended, but food sucks)
11. Visit Mt. Bonnell (great place to take a chick)
12. Tubing along the Guadalupe river
13. Visit Gruene TX (South of Austin), Gruene Hall, etc.
14. Go to Driftwood TX (SW of Austin) and visit Saltlick for great BBQ
15. Countyline also has great BBQ
16. Live music at Antones and Steamboat, etc.
17. Skydive in San Marcos TX (South of Austin, on I-35)
18. Go to Enchanted Rock State Park near Fedricksburg
19. Visit Perdinales Falls State Park
20. Watch the bats on Congress bridge
This is just some of the things you can do... there is actually alot. You can also check listings for concerts, as Austin is the "live music capitol of the U.S."