For the last 20 years we have been camping and 4 wheeling on this island called Bois Blanc Island. I'am in to road biking and never thought about going mountain biking there, this past time I borrwed a freinds mountain bike and had more fun there than I've ever had in a day. It's 25ish miles around, the trails are 4 wheeler and old logging trails. The island is next to mackinac island in michigan. There no fee, you camp on the beach, no camp ground, no bathroom, and the lake and beach are all to yourself. If you try going, there's nothing there that can hurt you, no bears or sunks, and you will need 4x4 if you plan going anywhere close to off the main dirt roads. Don't go without a car, it can stom really bad at night. If you're to hardcore for camping there's houses for rent and one small bed and breafest. The only things you have to watch for are sharp rocks that will pop you're bike tires and bring a map of the island (bought at the store when you get off the boat), an air horn, and a lighter. While you mountain biking there are, abandoned houses, cabins, rail roads and sawmills, all just sitting wayyy out in the woods, full sized tree's growing though them. In the 1800s it was all logged and prisners where sent there from mackinac island and all their buildings are still there, abonded in the woods
If anyone acually read all this the ferry is cheap and you can bring your car or truck for cheap.
*Their website http://www.bois-blanc.com/phpBB2/portal.php