Here's a serious response:

If I get too horribly cramped from sitting in economy class for fourteen hours going from JFK-NRT, I'll sometimes walk/march in place in the one of the toilets for a few minutes if there is room. It doesn't bother the other passengers and doesn't make me look like an idiot, so it's a good arrangement.

There is not going to be anywhere, on any plane, save for maybe Air Force One or some similarly decked out one, where you can do any substantial walking and certainly not anywhere where you can do pushups.

My advice: sit in your seat, stretch when getting up to use the bathroom, and maybe stand around in the back looking out the window for a minute if you want to get some blood flowing. But don't risk creating an incident (or at the very least making a fool out of yourself) by trying to do pushups or jogging around the plane.