From now until the first hard frost (usually sometime in October), it's raspberry season and the Ann Arbor area has some great pick-your-own options. I recently went to the Berry Hill Farm in Dexter (12835 N. Territorial Road) about a 20 minute drive from downtown AA.
The farm is basically run by an older couple out of the garage of their house, with the berry field being their backyard. That said, don't expect a small patch of raspberry bushes. The farm is sizable and offers five different varieties of raspberries as well as blackberries. My favorites happen to be the Dinkum and Jacelyns.
The picking conditions are great right now and I recommend checking out the farm. All varieties of berries are $2.75 a pint. It can be a bit buggy, but they also supply bug spray!