Other than going to the gym, the form of exercise I have used the most is what I call the "Writer's Block Routine". I start to write something, pace around the house, write a bit more, do lunges, watch TV and lift weights, roll a few sentences around, do some starjumps, etc. I had a chin up bar that was useful because I'd just do a couple of reps every time I walked through the door. This also works if I'm cleaning the house.