Colonising Space/Space resources
Resources within the solar system are mind-blowingly huge both in terms of energy and matter. There are planets, many moons and uncountable asteroids and comets.
In space the sun is always shining and there is no atmosphere to attenuates it glare. Solar power is the obvious way to go in, at least in the inner solar system. Either photo-voltaic for electrical power or solar furnaces for material processing.