As a theoretically unbiased non-Michigan resident, I'd heartily recommend a trip to Mackinnac Island (pronounced Mackinaw, if you did not already know). Even if you stay off island, you can fly to the island for the day (unless something has changed recently). Bicycling around the island is great. The Grand Hotel (the hotel used for the movie Somewhere In Time, if you ever saw that) is great. My wife and I had a great time. It was in the mid-eighties, so things could be different now.
As a child, I loved Greenfield Village in Dearborn, MI. For more, see this web site
http://www.hfmgv.org/ . I have not been there as an adult, it may be a tourist trap, or maybe have changed a lot, but it was a BIG highlight of my childhood.
Of course, I liked visiting the Kellogs factory in Battle Creek, so you see how I am (or was). For more info on that, and other sites in MI, see this Michigan travel site:
http://www.travel.michigan.org/detail.asp?p=G15930.
Enjoy,
Kevin