Some airports do have gyms, although they're not free. Anything is better than staring at concrete walls for hours at a time. Do you fly much?
No, thankfully I don't fly much. About once a year I have to fly somewhere on business, and I can't remember the last time I flew for any other reason. But I'm pretty good at amusing myself in boring places. I have a longish train commute every weekend (real home is in a Philly suburb, job and weekday apartment are in Manhattan), and train schedules and delays resulting in missed connections make for a fair amount of time spent in train stations. In the Trenton station, one's options are 1) read, 2) catch up on e-mail via Blackberry, 3) make conversation with other travellers, or 4) stare at a wall. I do a mixture of 1-3.