I’ve always been incredibly interested in the idea of harvesting energy.  There are so many small outputs of mechanical and kinetic forces in routine life — from doors closing, stomping of feet, knobs turning, heat from exercise, heat from cooking, etc. The collection of these outputs seems like it would something incredibly lucrative — albeit difficult to implement, but at least something to aim for.  DeputyDog recently highlighted a collection of techniques and technologies focused on such a goal.  For some green inspiration, make sure to check it out.