You probably have time to go downtown. Train ride from ORD to downtown is fairly lengthy (45-60 minutes), you'll need to allow plenty of time to get back to ensure you can get through TSA, etc. From the train stations where the blue line delivers you into downtown, you're relatively close to a lot of stuff and cabs are relatively cheap to go from the L stations to, oh say, the Willis (Sears) Tower, the Art Institute, the Field Museum, etc. Also, plenty of good food down there!