I was imagining a little rat-sized treadmill... but I guess that's essentially what a wheel is ^_^
Even my non-wheel-runners get a lot of exercise just by running around during free time, and they all like to climb around on stuff in the cage, which I guess its exercise. As long as they're getting good food that isn't too fattening, and run on the wheel or run around during free time, and have enough room in their cage, they should be nice and healthy!
