I've been traveling around to various Forest Preserves in the Chicagoland area and have found Lake County to have some of the niceset, most manicured parks around. Rollins Savanna Forest Preserve, being one of our favorites, is 1200 acres of mostly open, rolling prairie and a few ponds with 5.5 miles of trail that meander about making two separate but connected loops. The trail is the size of an old forest road and graded for running, biking and cross country skiing. Rooney and I typically head out there in the middle of the afternoon, when there aren't too many folks, though I can imagine it gets pretty crowded during peak hours and days. As far as the hiking goes, this is pretty tame and an adventure I'd piece together with another Preserve to make a full day.
Also, the ponds are a haven for several species of birds, so keep a look out.