<------ Former Austinite
Let's see, the green belt was my favorite part about Austin. If you wander off the trails and climb up the rock walls you can find a small colony of hobos to hang with. Some very fine folks live back there full time... Possibly my brother is one of them at the moment.
The Cathedral of Junk in south Austin is amazing and is owned by wonderful people. Go have a beer with them when they're open.
http://www.roadsideamerica.com/story/7816
This is a pretty important part of living in Austin:
http://www.showlistaustin.com/
If you're interested at all in improvised experimental whatnots, the Salvage Vanguard Theater is important:
http://www.salvagevanguard.org/index.php
For food... Ruby's BBQ near the UT campus is good.
http://farm1.static.flickr.com/201/4...e6bc9b.jpg?v=0
You can get some wicked bad ass Chinese food at Ding ho(sp?) in north Austin.
I tended to stay away from sixth st., but if you must go Jackalope and that one place across the streetish from emo's with all the heavy metal dudes that I can't remember the name of are the places to be.
I'll think of more stuff later.