TIL Floating cities above the clouds of Venus may be our best bet for becoming a two-planet species. Conditions there are so similar to Earth a human wouldn't need a pressurized-suit, the gravity is similar and transit times are shorter than to Mars.

http://www.science20.com/robert_inventor/will_we_build_colonies_that_float_over_venus_like_buckminster_fullers_cloud_nine-127573