Detroit looks its best around Christmas time. You will want to stay out of the city proper, but still lots to do and see.
Consider Dearborn Village. It has an early days theme, and Ford Motors history. You can go to cider mills, and munch a fresh donut, washed down with cider that you watch getting made.
Skiing is available at Mt. Brighton (small, but really convenient). Do some cross country skiing at several of the golf courses.
Agree with Frankenmuth. Gotta have one of their famous chicken dinners, and stroll through the year round Christmas shops.
Dress warm for December. Days are often pretty sunny and bright, with clear air, and fresh snow.
You will not be too far from the Walter P. Chrysler museum. You can see the first Vipers as well as some pretty cool prototypes.
Enjoy your trip.