Our tour of Chicago took place over five days. Each day focusing on a different facet of the city. One of the best ways of getting an overview of the city was literally getting an overview. The Willis Tower (formerly the Sears Tower) and the Hancock Bldg allows you to scan the entire city from the observation floors and in the case of the Hancock Bldg form the dining room on the 95 floor.
In addition to the spectacular views, both buildings offer exhibits discussing what you see outside with an explanation of the history of the city, as well.
At least one of these buildings is a must on any visit to Chicago.