Kickapoo State Park (27-30 mi)
Address: 10906 Kickapoo Park Road, Oakwood, IL

has a variety of trails, all moderate to easy in terms of difficulty.  The longest is a seven mile out and back trail that takes you through prairies, woods, and a few hills.  There is good canoeing, picnic tables, and open areas that make good places for hanging out.  You can also rent horses and go mountain biking. 
http://www.stateparks.com/kickapoo.html