If you're coming from the south, I suggest going to the South Kettle Moraine Forest first and ride the John Muir trail system (lots of twisty single track) then stop by Backyard Bikes in LaGrange for lunch (about a mile from the trail head). Keep going north and stop to ride the Nine Mile Forest trails in Wausau, WI on your way to the Chequamegon Forest in northwest WI. Besides the MTB trails, you can also ride the snowmobile trails and forest roads. Up there, the snowmobile trails become ATV trails in the summer as opposed to the southern part of WI where the trails are mostly on private land. The government forest lands up there are HUGE. You can drive for 45 minutes at 80 mph on US 2 across the Upper Peninsula of Michigan and not see a single house. (Of course, don't actually drive that fast lest you meet up with a deer, bear, or rare moose!)
In some places you'd be hard pressed to find a station on your radio!
I have a video of this year's Chequamegon 40 mile race on YouTube, this is one of the snowmobile trails: