Since we have lots of threads about NYC I think Chicago needs some love.
I'll start:
The skyline. Best in America!
Wrigley Field...a baseball shrine!
The Wrigleyville/Boystown area
El stations right across from the airport (Midway) or actually in the airport (O'Hare), so there's a cheap way to get downtown from there.
The Printers Row Book Fair
Taste of Chicago
Grant Park
All the museums
Sitting in front of "Nighthawks" at the Art Institute