Originally Posted by Efrem
At ORD, you might take the shuttle train to Terminal 3 (if you're not already there), then walk through the tunnel to the Hilton. On your right, directly opposite where you would turn left into the hotel, is an airline memorabilia shop where you can easily browse for more than the time you'll have. If you want an Eastern Airlines golf shirt, that's the place to get it! It means going landside and clearing security again, but you should have time for that.
This wouldn't be useful for everyone, but I know there's a workout facility at the Hilton, and I believe I remember once reading that one could purchase a day pass.
On a more obvious note, you could always get into one of the airline lounges to relax. The only reason I bring that up is that I enjoy sitting in the American Airlines Admirals Club, at the Southwest corner; it seems to have the best runway views, though I'm certain a more haggard roadwarrior can suggest an even better spot.