The garden at the Michigan League (U of Michigan) is beautiful. There are rooms for overnight guests upstairs in the League (though they might already be booked)~ great for out of town guests. They can walk to the wedding and everywhere else in town.
There are tons of locations including any of the numerous country clubs, the Arb, Mattheai, Ann Arbor City Club, any of the metroparks (Hudson Mills/Dexter, Delhi, Dexter Huron) Eastern Michigan University has a lovely location for outside, Concordia University, Gandy Dancer, Cobblestone Farms, Gordon Hall in Dexter (really worth checking out), River Birch Lodge in Hartland (WOW). THere are just a few!
I would recommend The Mansion at Pine Knob. I still have the client review low resolution proof version of that wedding video online if you would like to view it. It is probably good for weddings with up to a hundred people but will also work nicely for as little as twenty. You will see in the video that the colors are magnificent with a breath taking view of the surrounding area.
My name is Kathy Byrum, and I am a wedding planner. My company is Dream Wedding and Event Planners, and I have planned and coordinated numerous weddings outdoors in the Ann Arbor area. I can help you put together a beautiful outdoor wedding on July 29th. Attached is a picture of one of them. You are getting a late start, but I have the resources to get it done. Call me if you would like to talk at 734.582.0200. Have a great day!