One of my favorite places in the lower peninsula is close to Flint and if you enter at Port Huron you'll be passing by is the Crossroads / Huckleberry Railroad
Whenever I passed through the area I would go there with my children. The village is great and the train is authentic even if a half continent away from home.
The other suggestions I have would be Traverse City, Fort Michlimackinac (see
Colonial Fort Michilimackinac - Mackinaw City, Michigan
) which is within sight of the Mighty Mackinaw bridge over the straits at the north end of Lake Michigan.
If you just do the tourist things you'll be able to kill a year or two of really great places to see.
Soo Locks tours http://www.soolocks.com/index.phtml
Shipwreck Museum - Paradise Michigan (west on M-28 from I-75 about 30 miles IIRC) http://www.shipwreckmuseum.com/
Great Golfing in the area around Gaylord Michigan
Super fishing for Bass in the area around Hessel and Cedarville areas
Mackinaw Island is worth a day or two all by itself.http://www.mackinacisland.org/
If you get to the west part of the upper peninsula there are the painted cliffs at Munising http://www.picturedrocks.com/Home.php
and some beautiful scenery all along the south shore of Lake Superior.
That part of the states I spent a lot of time in when I lived back east.
Way too much fun to be had there.