Windy City Hikers.
Great group of people and of all ages and backgrounds.
Check them out through
Real groups make a real difference - Meetup.com
Incidentally many members live in the Lincoln Square neighborhood. A good neighborhood also. They have the Old Town School of Folk Music and the annual Chicago folk and roots music festival that they just got through.
Anyways: Windy City Hikers. Laid back, outings all the time. We've canoed on the Kishwaukee near Rockford, the lower Fox, and have gone tubing on th Tippecanoe in Indiana.
Chicago is not exactly a city of nature lovers and outdoor enthusiasts, however because its so big and diverse there are thankfully many opportunities to meet like-minded people.
Good luck! Check out the website.