I'm always glad to see people volunteering for trail work and nature areas - thank you! I have taken a couple of backpacking trips on the Manistee River/North Country Trail Loop (in Manistee National Forest) and it is a great area. Here are others I want to try, based on the book "50 Hikes in Michigan" by Jim DuFresne:
South Manitou Island (long ferry ride required)
Highbanks Trail (in Huron National Forest)
Jordan River Pathway (in Mackinaw State Forest)
Chippewa Trail in (Negwegon State Park)