I live near flint and if you go to the Mounds you need to tell them you are a resident and be prepared to tell them a city,town, ect in Genesee county if not a day pass will cost you more.
The mounds to me is more a of mud bog pit now. Lots of 4x4s and ruts. Last time I was there it was taken care of quite nice at the beginning of the season. The real area, where the "Mounds" hills are, is closed now because of all the accidents.
Don't know about any other good riding spots except organized tracks and raceways:
MI MX Tracks
My friend has a quad and no one to ride with but me. He wants to go all the time, but has no ride. I just do not have the money anymore.