Solar Energy - The Sun is huge in comparison to Earth. Can barely see Earth when comparative sizes are used. We could fill the lit surface of the moon with solar panels and the astronauts would have something visibly useful to do in space: setting up the solar panels, collecting the batteries, and/or building an infrastructure to transmit the energy to Earth. Staring at that huge ball of potential energy, it seems incredibly ridiculous and near sighted to not exploit all that free power.