Jenny,
Michigan is a great destination. Unfortunately, there is so much that I don't know what to recommend unless we have some input.
Sleeping Bear Dunes and the Ghost Forest are all neat and can kill several days. You will want the Jeep for the Pierce Stocking Scenic Drive (don't miss that).
As you come up from the south, if you are west, look up the Gilmore car museum. If the kids are interested, you should also stop at the Kalamazoo Air Museum (Zoo). (It used to called the air zoo because they housed a collection of Grumman Cats.)
If you are coming up east, do not miss spending at least a day, maybe two at the Henry Ford Museum and Greenfield Village (attached, but two items). Each of these can kill a day if you have the time.
Let us know what you are most interested in and further advice can be more directed.
Matt
