You'd love Austin's Hope Outdoor Gallery/Park at Castle Hills.
My first trip out there was the middle of the winter, and it was absolutely empty. My most recent trip was a beautiful spring day, and there were (kid you not) maybe 200 people crawling all over it.
Some samples of the stuff I thought was pretty slick:
1. The whole place is fair game. It attracts a ton of local talent, and a ton of disturbers of the actual art.
2. Some folks are damn talented.
3. Some folks I want to ask what they're taking
4. Some folks are just very... Austin.