If you are marking events on your calender, make sure to add these two events:
1) The 26th Annual Harvest Moon Cruize - Sept 14 - Lamar Ped Bridge - 2am
This happens once a year. I caught it last year (by accident). This is as 'Austin' as you get, folks. There will be 400-500 riders leaving the Lamar Ped Bridge at 2am and going on a leisurely ride around Austin. There are frequent stops where everyone parties and dances. People haul beer, kegs, coolers, sound systems and everything else you can imagine. It's really quite amazing. Ride usually finishes somewhere downtown about 5am. I stumbled upon this ride last year (quite literally) and it was really amazing. Try to plan on it. Note - that day is a Sunday - ride starts at 2am (you can also look at this as really late Sat night). I know it's late, but it is worth it.
2) Tour de Fat - Oct 18 - Festival Gardens 9am-4pm
Crowder and I went last fall and this was eaisly the coolest thing I did last year This event is sponsored by the New Belgian Brewery (Fat Tire) and is a very weird blend of a bike parade/carnival/revival. The brewery brings all sorts of bizarre bikes. Beer, live music, and a short, very fun bike ride. Strange costumes are highly encouraged. This is a good event to bring the family as well. Seriously - don't miss this one.
This event is really hard to describe - youtube is worth 1,000 words - http://www.youtube.com/watch?
If you are looking for more bike stuff to do in town, check out this blog and subscribe to his mailing list. www.atxbs.com (Austin Bike S*it)