I recently had a 4hour layover at O'Hare and did not want to spend that time just sitting at the gate or eating, so I took a long walk from Concourse B to Concourse L and here are some of the many things I saw which could keep you entertained during a stay at the airport. (Some of these are especially good for kids.)
1. Dinosaur skeleton at Gate B-8
2. Neon Light Show--walkway from B to C
3. Art by high school students in the gallery 37 program: painted benches and the Millenium Chandeliers which were made by the students under the guidance of the Chihuly Studios--walkway B to E/F
4. Display of Wildcat F4F-3 plane and exhibit about Lt.Commander Butch O'Hare--walkway C to E/F
5. Verizon Wireless Kiosk: just fill out a card with name and address and you can use their laptops, hassle-free and no charge. Located near the F4F-3 plane
6. "Kids on the Fly" playspace by the Children's Museum of Chicago--walkway between E/F and Terminal 3
7. Chicago Photo Exhibit--walkway G to H
8. "Chicago: The City that Inspires" photo exhibit--end of H/K concourse
Hope this helps you while away the hours at O'Hare!