I agree with a lot of what is posted here, you can cross the Mackinaw Bridge, but everything around there is just tourist trap. Do stop at Saute Saint Marie and watch them flush a ship or two through the locks. Several good campgrounds close by. Lots of tourist stuff too, but the locks are neat. Drop someone near the visitor center as soon as you can to see the schedule (sort of) on the wall so you know when to come back.
While there, the River of History Museum and the Valley Camp museum ship are both good, but don't bother with the tower unless it is cheap to add.
What they are saying about Henry Ford and Greenfield Village is true. The Henry Ford alone would be a one day and 100+ (diversion rate) museum. Greenfield Village is a assembly of historic structures, but unlike the historic villages of the east coast, they are not all period, but most are historic.
Keep a running log or it will all be blur in December.
Matt