I lived in Austin for a couple of years around 1990. It's the wrong season, but Hamilton Pool south of town is pretty awesome, with a waterfall and a grotto beneath. Also, if I were going back, I would go to the original Salt Lick, which is in the middle of nowhere south of the city, but I understand that now it's a franchise with a couple of other locations. Their barbecue is fantastic and it's all you can eat, but the original site is (or at least it was) in a dry area so BYOB and they'll let you drink there. Have fun on Sixth Street.