Do any of these appeal you to you?
rollerskating
rock climbing (Austin Rock Gym)
concert*
I understand that Austin has a great music scene, and is a big college town. You might be able to find free or very low-cost concerts on one of the local college campuses. And since most college students are under 21, your and your friends' ages won't be a problem in terms of getting in. If that sounds like fun, check the websites of the schools near you - it'll probably be something like "calendar of events."
Fishercat
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