Once you get past the point of "energy I would have expended anyway for exercise/to relieve boredom" you run into the fact that humans just aren't a very good w

Once you get past the point of "energy I would have expended anyway for exercise/to relieve boredom" you run into the fact that humans just aren't a very good way of converting resources (food and the energy/minerals needed to grow it) into mechanical energy. The modern food system is woefully inefficient at converting primary energy into food calories and the human body isn't all that great at it converting food into mechanical work. Useful amounts of heat are even more elusive. Toasting a slice of bread might take a half hour of vigorous exercise. I'm pretty sure it would take more energy to heat a jacuzzi than one could extract from a dozen people showing off in front of it.