I was just in Traverse City... took a weeklong tour of Michigan, ending up in Sault Ste. Marie. On the way home (to Metro Detroit), we rode the Black Lake and Red Bridge trails (Cheboygan county, near Mackinaw). It was nice, and since the weather wasn't great, there was no one else on the trails.
Not sure exactly where you live, but if you're up north or in the U.P., here are the state trails: DNR - ORV/ATV Trail Maps
. You'll want to stick to the ORV Routes (the red double dashed lines with the Jeep symbol), since the ATV and bike trails are too narrow. Of course you can also use the seasonal roads (the gray dashed lines).
Down south, there's The Mounds in Genesee county near Flint (The Mounds Off Road ORV Park, Flint MI Genesee County
) and Bundy Hill in Hillsdale county near Jackson (Michigan Off Roading at Bundy Hill Off Road Park - Jerome, MI (4x4, OHV, ORV, ATV, Quad, Motorcycle, Jeep)
). These are ORV parks, which mostly consist of mud pits, though Bundy Hill has some nice trails too. Finally, there's of course Silver Lake sand dunes, which I haven't been to and probably never will since that's not really my thing.
IMHO, the trails up north are the way to go if you don't mind the 3+ hour drive each way. I picked Black Lake last week since it was one of the further trails from where I live that was along our route, which means it's one I wouldn't easily get back to any time soon. I'll post some pics as soon as I finish going through them.