[TrevorJ]

>We know that rodents enjoy running wheels because they use them voluntarily.

I can think of a lot of behaviors that humans engage in voluntarily that we would not call 'enjoyable', so I am not certain we can state this categorically.

I totally agree with the rest of your statement though, and I would add that people tend to overlook variety as well. Enjoyment is so key, and we tend to enjoy things that are novel. Relative to the cost of a gym membership, it's pretty reasonable to pick up a new activity/active hobby every so often to add to the mix. A plastic kayak, some old skis, a tennis racket. If you are saving 50-100 bucks a month by not paying for a gym, allow yourself the luxury of some new (to you) equipment every couple of months and keep things fresh.