You covered state parks and the national park service nicely. Michigan also has land designated as federal wilderness areas. Here's a link to the USFS Wilderness Area page that covers the Sylvania Wilderness, McCormick Wilderness, and Sturgeon River Gorge Wilderness areas. Supposedly, this area is the largest roadless area in the state.
"As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life - so I became a scientist. This is like becoming an archbishop so you can meet girls."