If you like this type of stuff:
Museum of Science and Industry - I've enjoyed this both as a kid and adult
Can check out Millennium Park and The Bean.
Walk/run/bike/other the Lake Shore Drive path.
Walking downtown can be fun, just to check out the skyscrapers, particularly if you don't get to see big cities often. If you have money to burn get reservations at The Signature Room, Cite, or another restaurant with a nice view.
A million normal good eating places. A Chicago style hot dog is fantastic, even from many random vendors. Pizza from Gino's East or Lou Malnati's are my favorites, but there are several others that I'm sure are raved about also. Sausage in a Chicago deep dish is great, as they press an entire layer of meat into the pizza. And Lou Malnati's has the only non meat pizza I'll eat, "The Lou". Find a place with Italian Beef and hit it up. Get an Italian Ice for dessert. Better than a Beef is a Combo, which is an Italian Beef with a sausage/brat stuffed into it. If you leave the city to the North suburbs I have a coupe more restaurant recommendations.
Plenty of bars, shows, and concerts to search for if you are inclined.
I'll have to put all this stuff on my list to check out, thanks Slatch.
I'm gonna check out the Signature Room and gonna look for the best the Italian beef and Italian Combo, they look amazing.
So far I've got Tony's Italian Beef and possibly Al's beef, I'ma do some more looking though,