create (rather, modify existing designs of) gym machines that generate electricity, sell to metropolis gyms. treadmills, stationary bikes, elliptical trainers and stair-masters are easy; what's slightly harder is the weight-machines. how would you have the user generate more electricity, as the resistance is increased? in other words, how do you make sure the increase in resistance is achieved by generating more electricity, rather that by adding more weights?
starting point: http://en.wikipedia.org/wiki/Dynamo
makes all the sense in the world, right?