At the risk of sounding cheesy, I would work on something that helps soothe or at least mitigate the destructive effects humanity has on the planet, ecosystems etc. This could be anything from renewable energy, self-sustaining houses, solar powered cars, arcologies, biodegradable or fully recyclable materials and so on. It's a far cry from the web stuff I am working on now. I would definitely need to acquire some new skills first (which would not be an issue if I don't need to worry about money).

Also, education. Things like Khan academy are amazing, but not yet accessible to everyone in the world. Thinking about that, working on internet access for all seems like a pretty noble cause too.