Being that I grew up in the city, and was born there I feel like i have a pretty good grasp in Chicago.
Def go see Millenium park, the bean, shed aquarium is really really cool, museum of science and industry is really really cool, and I think Mythbusters in there right now. A cubs game is a must. They suck so bad right now you can go right to the ball park and get cheap bleacher seats. Go to the top of the Willis tower and walk on the sky deck, which is basically a glass overhang..its pretty neat.
The best deep dish pizza in Chicago is Lou malnati's, hands down. Go to Hot Doug's on California if you want to eat at the best gourmet hot dog joint of your life (duck fat french fries). There will be a minimum of a 30-45 min wait, but its well worth the wait. The best beef in the city is at Al's beef its amazing. You should go to the one on Taylor st, get some beefs, and the walk over to Mario's Italian Ice stand, and get the best Italian Ice you can get.
Take a boat tour at Navy peer. Go to North Ave beach for the day, and party at Castaways Bar, which is the best beach bar in the City.
Bike along the massive bike path along lake michigan. If your old enough go down to Rush and Division, tons and tons of bars and restaurants. It is hopping every night during the summer.
The taste of chicago is good, but expensive, and I don't really go anymore because the quality and quanity of food establishments have gone down in recent years.