Let me tell you of the wombat massage.
Firstborn will be six in June. He is the inventor of the wombat massage. The wombat massage is... well, as a treatment for sore backs after a long day at work, it's one of the finest known to man. (Not "Man". Just "man". A man. Specifically, me.)
The first thing he has you do is lie down on your belly. Any surface will work, as long as it's flat and reasonably comfortable; I've received wombat massages on the couch, the bed, and the living room floor.
Once you're laying down and stretched out, Firstborn steps onto your back. He puts his feet down at the point where the small of your back connects with your pelvis. He puts his hands down on the back of your shoulders, so he's now crouched on all fours on top of you. Then he rocks back and forth, like a wombat waddling in place.
This won't still work by the time he turns sixteen, or even twelve, but right now he's the perfect size and weight for it - and it works really, really well.