Born chicagoan, I've gotta throw in my 2 cents. There is an unbelievable amount of things to do. All of the below aree either right downtown, or within 10 minutes. Heres the short list.
Food:
--Mr. Beef: Best italian beefs in the city.
--Chicago Chop House: Very upscale steakhouse right downtown.
www.chicagochophouse.com
--Rosebud, on Taylor St.: Amazing Italian food in Little Italy! Mind your manners while there, its a known "family" business.
http://www.rosebudrestaurants.com/therosebud.htm
Culture:
--Art Institute of Chicago: Some of the worlds best known pieces are here. Located on Magnificent Mile.
http://www.artic.edu/
--Museum of Science and Industry: Worth checking out. U-Boat not open now right now though.
www.msichicago.org/
--Navy Pier: Outdoor.Indoor Pier that stretches out Lake Michigan. Huge ferris wheel, stores, IMAX theater, and restaurants with beer gardens.
www.navypier.com/
Nightlife:
--Rush and Division: A block of bars and clubs!
www.rushanddivision.com
--Many bars and clubs aorund city: same company owns all of these very cool properties around city. From college pubs, to all out mega dance clubs. Find your taste.
http://www.aceplaces.com
--Signature Room: This is a lounge on the 95th floor of the Hancock Tower that offers 360 degree views that extend roughly 200+ miles out. AMAZING at night with that special someone.
http://www.signaturelounge.com/
If you're gonna be in other parts of the city or 'burbs I can get you info. if you let me know where at. Enjoy.