If your just looking for something to do to "Stay in shape", I'd suggest finding a gym that offers boxing classes. Boxing for an hour or 2, two days a week will keep you in shape. I personally work 6Am to 6PM Monday through Friday. I wake up at 4AM, don't get home till 7PM. Then I'll stay up till around midnight or so before going to sleep, so I myself only get about 4 hours of sleep also. I've been training in Martial Arts for quite some time now and let me tell you, I take classes 3 days a week and I can run circles around most people. It may not build muscle, but you gain strength and stamina, and it's a hell of a lot of fun.
Don't k ow if boxing OR Martial Arts is your thing tho. Just my .02