There is a nice selection of trails right off of I75 at St Helen - exit 222 I think. Take M76 north a couple of miles to "downtown" St Helen, then take a right at the only stop light in town (Airport Rd). A couple of miles east will bring you to the St Helen Scramble Area.
Alternately, getting off at the same exit but heading back under the freeway a couple hundred yards will bring you to Old M55. A left will take you east towards West Branch. All along both sides of the highway are legal 2 tracks. If you continue east to Dunham Lake Rd and head south there are several miles of scenic ORV Routes that form a loop that cross Dunham Lake a couple times.