Or go to the Millenium Park area where the Planetarium, History Museum, and Navy Pier are within walking distance of each other.

My family's favorite was Science and Industry but the Planetarium was a very close second. The Aquarium can be skipped.

The Willis Tower is cool but you might like the Handock view better. You can hit the Ghirardelli's shop and Hershey's shop at the top of Michigan. Catch a show in the evening at the theatre district.