Back in the days before I got into shape, whenever I found myself waiting for an airplane flight - and traveling alone - I would find a seat, preferably with a power outlet nearby, and either read a book, do some writing on my laptop, or play computer games.
Nowadays, I use the time to boost my step count for the day. During my layover in Salt Lake City yesterday, I deplaned in Gate B, walked to end of Gate C, back to the connecting hallway, all the way to the end of Gate D, then back to the hallway, to the end of Gate E, and back. I went back and forth along Gate C so often that I was surprised when the Delta credit card guy kept trying to get my attention. I'm not sure if he realized that the same guy kept declining interest with the same explanation: "No thanks, I've got enough plastic."
Walking was better than nothing, although it was a bit of a pain to lug around my "carry-on" bag, which contained my laptop, power supply, and other stuff I didn't leave in my checked bag. During my pacing, I noticed that one of the airport vendors offered massages. For $65, you can get a 30 minute full body massage.
That got me thinking: someone should start an airport business that offers treadmills, free weights, and maybe stationary bikes, along with a quick shower facility. You can't really go running in the airport, because (1) TSA probably wouldn't like it, (2) it's too crowded in spots, (3) you can't run effectively with your carry-ons, and (4) you can't leave your carry-ons in a fixed place. With a treadmill, though, you could just leave your bags to the side where you can keep an eye on them.
I don't imagine there would be a big audience for this service, but I think there would an intense and dedicated one. Obviously, it would work best in the "hub" airports where more people have layovers for connecting flights.