I know this has been done before but might still work fine in this mod: animal powered rotational power generation. Put some animal atop a treadmill and watch them power your contraptions!

A more wild and definitely silly suggestion: slimes generate rotational power by constantly jumping on top of a piston-like contraption. Could also allow players to use it, as an alternative to the hand crank.

@simibubi Also, different animals could provide varying values for speed and torque. Chickens, for example, would provide quite a bit of speed but not much in terms of torque while cows would be the opposite and so on. If you feel like adding some trade-off while also giving some purpose to all the food that can be gathered by other contraptions, maybe add a trough that needs to be placed in front of the treadmill and provided with the food item that the used animal requires so that they would be willing to even start running in the first place. Alternatively, the trough is optional and simply boosts the speed and/or torque provided by the animal.

Thanks for your suggestion, it has been added to our public submission sheet.