There are a million of ideas that float in my head for this one (I particularly like aDios's idea!). Schools, hospitals, libraries. . .
But that's all stuff that I'd want when things are already all well and good. After seeing tsunamis, hurricanes and earthquakes take the homes and loved ones of so many people, though, all in under a year's time, what I'd want to do now, if I could, would be to build their homes again.