I read the books and leave any I finish for the next traveler.

I set the alarm on my watch or cell phone and take a nap if I get sleepy.

Some airports are good for walking and you can even find out how far it is to speed walk around the terminal. I think it's 1.2 miles around DTW. Walk briskly for 45 minutes to an hour, and there's your exercise. Don't get in people's way, though, some people actually have to be somewhere.