Hinckley Reservation is a mountainboarders dream, at least by Ohio standards. You'll find miles of singletrack trails going up and down the hills through the woods of this park, although I wouldn't suggest trying them without a brake unit on your board as some can get pretty steep.
Beginner to intermediate riders will find the "Coasting Hill" a perfect long grassy hill to work on your turns and control.
My favorite spot is by the Swimming Area. When you are facing the dam from Bellus Road go to the left side by the bath house and start from the top. You'll find several different lines to take with varying difficulty and obstacles to negotiate including some natural berms, jumps, and dropoffs that can be jumped as well. The area can get crowded during hot summer days, and I wouldn't recommend riding at these times as it could put both yourself and others in danger. Evenings and cooler days will usually find the area practically deserted.
One final area to check out is Whipps Ledges, but only for the advanced rider with a handbrake.