How about we just continue to have robots colonize Mars? Just get increasingly sophisticated machines down there to build up the infrastructure. Maybe in 200 years they might have built some nice domes and have earth like gravity/atmosphere inside that animals and plants can be brought to Mars. And maybe then some humans might see it as an attractive destination.

In the mean time, we can colonize difficult places like under the ocean, the Sahara desert, Antarctica, maybe even the moon.