Honestly there's not a whole lot for public riding around much of Minnesota period. Unless you know someone with a lot of land to ride in is tough.
That being said, check out spider lake and the iron range. Both have dedicated orv parks that vary in difficulty depending on what you want to do, mud, rocks, trails, etc.
Dirtopia.com is a great website with a good list of trails and parks across the us. Check it out and see if Wisconsin has any good ones. Allot of the trails listed have info on type of terrain and level of difficulty.