Johansen's Pumpkin farm, in Bolingbrook has a lot of fun and free activities.
http://www.johansenfarms.com/
Places like Bengston's has a lot of activities, (including pig races) but is much more commercial (and can get PRICEY!)
http://www.pumpkinfarm.com/
Also, Round Barn, in Manhattan, IL, is a beautiful, old fashioned, property, managed by the Park District. Their web-site is currently under revision but they did host some special events last year so you may want to call the park district, directly, to inquire.
http://www.manhattanparkdistrictil.org/home.html