An interesting approach would be to send an automated ship builder robot fleet to dismantle an asteroid and board the fully furnished ship when it makes another close approach, and then boosting it to its final destination.

I'd love to see a scenario where we'd build robot terraforming fleets that'd land on an uninhabited planet, make it habitable, complete with spaceports, cities, factories, crops (all permanently tended for robots), build a couple extra fleets and launch to their next most promising targets.

And then, for some reason, humans lose interest in all that and someone else wakes up to find a universe full of 60's futuristic utopian planets nobody ever lived in.