EAT!
Go to Portillo's and get an Italian Beef sandwich, best thing about chi-town.
If you want deep dish pizza, only go to Lou Malnati's and get the buttered crust.
Weber Grill restaurant. Every steak is grilled on a Weber grill, pretty darn tasty IMO.
Great local brewery (recently went national this year) called Goose Island.. 312 is my favorite (the yellow one, the green 312 is hoppier)
Sights:
Architecture. Some amazing structures downtown other than the usual Sears (refuse to call it Willis) Tower.
Navy Pier
Awesome museum/aquarium.
O'Hare is kind of in the burbs so if you don't have a car or don't want to take the train downtown, they still have those restaurants I mentioned in the NW suburbs. If you have time to just kill and you want to stay in your area, you could always spend a day walking around Woodfield Mall, one of the largest in the US.