I've taken several steps to combat my slip into Larda$$ness. I've dropped over 20 lbs. since Jan 9 doing the following:
First: at my crashpad I prepare several healthy meals that I keep in the pilot lounge refrigerators to eat at stopovers in ATL. I also keep several of those new foil-pouch containers of tuna in my bag. They've bailed me out several times. I also keep a shaker along with about six envelopes of Myoplex or some other meal replacement powder (MRP) in my bag plus a good multivitamin.
As far as working out I belong to Gold's Gym. I have a travel card with them that allows me to work out at Golds' that are in layover cities. The van driver will usually take you for a couple bucks if it's not in walking distance. It's also worth it to me to pony up $5 or $10 for a good gym; a few of our layover cities have them within walking distance.
If there's not a gym you can run, like captjim, or carry a jumprope.
Pushups, crunches, walking lunges; there are quite a few weightless exercises. Also, look into purchasing one of those rubber hoses that can be used for curls, squats, and presses.
Bottom line: If you want to do it you will find a way.
Good luck.