Everyone's heard of the Sears Tower; it's arguably the most famous landmark in Chicago. Right in the heart of the Financial District, this is an easy attraction to get to. Paying an $11 admission fee gets you up to the 103rd floor, to the observation deck. The elevator that takes you there does so in an amazing 45 seconds (!). Once on the top, you'll be able to ogle Chicago from every possible angle, thanks to the all-glass paneling on the deck. The price is a little steep for the 15 or so minutes you'll spend up there, but the views are extraordinary; you can see up to 30 miles on a clear day. This or the John Hancock is a must for Chicago visitors; residents might find it a bit too corny.
Be advised that lines get realllly long on weekends; a great time to go is early-evening on a weeknight. You might not have to wait in line at all. And remember to bring your camera--photo-ops abound! Another tip: you can definitely skip the 10-minute movie they try to show you before you ascend to the top; it's a complete waste of time.