Water Found Deep Inside the Moon
I've always been more excited about a Moon colony than a Mars one. 
I mean, compared to Mars, the Moon is ridiculously close.Many of the same benefits Musk seeks (backup for Civilization, excitement) for a fraction of the cost.Low gravity and atmosphere on a dome would be more complicated, but again, compared to Mars, it's right there.It would be a more humble and realistic first step, IMO.