I was born and raised in Michigan - there are some great ideas above on the UP. We are headed there later this summer. I also highly recommend the Petoskey and Traverse City area - lots to do there:
Rent a boat on Torch Lake or one of the other "chain of lakes"
Visit Sleeping Bear Dunes - one of of my favorite National Parks with amazing views
Drive the Tunnel of Trees near Harbor Springs
Take in the wineries on the Leelanau Peninsula with great views of Grand Traverse Bay
Golf on one of the great courses in the area
Have some great pie at Traverse City Pie company
Bike on the Little Traverse Wheelway
If you want to try out a high end Class A RV resort - we like Bay Harbor Signature and will be trying out Hearthside Grove this summer. Both are among the best in the country (but pricey!).
2011 Tiffin Phaeton 42QBH
2011 Ford Edge AWD
Readybrute tow bar