The closest I know of are the groomed cinder/sand trails at Richard Bong State Recreation Area. I've also been up to McCaslin Mtn Campground's trails and some Nicolet NF FSRs. Are there other WI trails/FSRs close to the Chicago area? I have yet to ride along the Mississippi much north of the border, but some of the roads out there in NW IL/ SW WI are unpaved. I'm looking for preferably trails or FSRs. Singletrack or ATV-width, like MI has, would be great. BTW...The only legal dirt more technical than unpaved roads I have found in N IL are at MX tracks, or off-road parks like The Cliffs Insane Terrain (Utica IL- US6). IN offers The Badlands Off-road Park, about 2-2.5hrs SE of Chicago, in Attica IN. Most of the park type of riding is only open on weekends, AFAIK.
https://dnr.wi.gov/topic/parks/name/richardbong/ Richard Bong Rec Area has some trails and is about 1.5 hours away.