Your snake does not need suplemental exercise. In fact, the act of constricting and eating prey requires quite a bit of strength and energy. Can you imagine tensing all the muscles in your body for 10-15 minutes at a time? And then proceed to swallow a watermelon whole with your hands tied behind your back.

Well obviously this is an exaggeration, but you get the idea. Your snake will probably not get fat even if you never take it out of its cage and feed it once a week. If you feed your snake an appropriate sized meal once a week, they will be perfectly healthy weight wise.