I don't do detecting, but I have been to Kankakee State Park many times. I would think Rock Creek (or is it river?) would be a great place to search. Are you familiar with it? It's directly adjacent and at the far end of the parking lot. It's the creek below the pedestrian bridge. I would think you'd find a lot of good stuff along the banks and the many unofficial trails. Good luck and let me know how it goes if you do it.