To steal an idea from sic-fi writer Kim Stanley Robinson (and others, I'm sure) we'll hollow out asteroids, some 10-30 km in length and build earth-like terrariums inside. They'll be fully self-sufficient little cities of tens of thousands of people containing housing, schools, agriculture, fabrication plants, health care facilities, recreation and even a proper English pub. Put a nuclear-powered engine on the ass end of it, give it a spin for gravity and away we go. We'll get there. It will take generations living inside these terrariums but we'll get there.
Sounds like an idea to spawn a million scenarios