Robert McClory Trail (Lake County, IL)

I'd like to mention that, while the Robert McClory Trail technically starts in Highland Park (on it's south end), the Green Bay trail continues south into Wilmette. These two connected paths are paved and crushed limestone (50/50ish). It's a virtually all flat, and is a great way to get from Chicago north, say to Wisconsin, or Illinois Beach State Park. Bonus is it parallels a very close Metra UP North line, so a train trip can easily combine with your bike ride. Learn more:

http://www.everytrail.com/guide/robert-mcclory-bike-trail-south