See http://trekohio.com/halloween/ for a list of Halloween events being held at Ohio State Parks. Activities include campsite decorating, costumes, trick-or-treating, bonfires, hayrides, pumpkin carving, haunted trails, etc.
We checked out ODNR’s Natural Resources Park at the Ohio State Fair (ongoing through August 3rd). Among other things, we got to see the staff capture ducks from a pond before dogs could be released to retrieve duck decoys. :-)