There is a lot to do other than the dranking and skin scene (FYI: you can do the dranking and skin thing during the day too):
Insectarium - Pretty cool. Real easy to get to and in the "heart" of most of the stuff you will see.
City Park - The park is pretty cool (Celebration in the Oaks at night: http://celebrationintheoaks.com/cito10/ currently $7). Also, has a Sculpture Park and Art Museum. The street cars can take you right to this. BTW, $1.25 one way for a street car ride.
Audobon Zoo and Aquarium - It is worth the admission if you are into this kind of thing.
Eat - the food is always amazing, no matter the time of day.
I know someone else suggested taking a street car up St. Charles, something I would also suggest, but if you do get off around the 4000 block and walk over to Magazine Street (2 to 6 blocks away depending on where you are). Once you are on Magazine just walk back towards downtown. There is all kinds of shops and restaurants that you can spend a ton of time in. Magazine has a real young, energetic vibe. I am guessing fans of Ann Arbor will like Magazine.