You could go on a private hot air balloon ride(this one has champagne on some of them- since you'll finally be legal and all)
http://www.austinballoonrides.com/
and maybe a gourmet picnic style type party or something with your closest friends/boyfriend. Not exactly wild and crazy but it's something that will lead to better memories than getting fall down drunk in Vegas and puking all over yourself(which is how I spent my 21st). I'm 28 now and in retrospect I really wish I would've taken a hot air balloon ride with champagne and had a nice picnic. LOL.
Or you could go to an Austin karaoke bar with PRIVATE rooms and have a little party there. There are quite a few private karaoke room places in Austin and all they have their own vibe- check them out before committing to one...some are BYOB and ghetto but hilarious and fun in their own way and some are a lot more upscale with nice food and drinks available etc.
Hope this helps...don't end up getting drunk on 6th street for your b-day...there will be plenty of weekends for that, it's your birthday do something special.