yeah, sure, but you can either terraform planets to make them more habitable for mankind, or you can start to build isolated compounds cut off from the outside world to live in.
as for mars, it could for example be possible to contaminate it with some sorts of bacteria who start to produce CO2 which in the long run would cause a rise in temperature. eventually even the icy pole caps will melt and water will be supplied.
there are dozens of scenarios like this and sooner or later mankind will try them out, i guess.
anyway, at first "we" need to establish a secure and reliable space station. hope i'll live to see the day when the ISS is really finished.