You should look into white water rafting and perhaps camping the night before or after you raft. My chapter (and other sororities on campus and at a neighboring school) go rafting for Bid Day and it is a lot of fun. Sure you're asking about a retreat but the idea is the same. Sisters spending time together doing something but not any programming. We have a camp fire, roast marshmallows, etc. and all the girls can work on their tans if they so desire.
A google search came up with
www.texasrafting.com and lists water near Austin - Roundrock if that helps.