I actually had a similar idea. The project that I was thinking of mentioning was one that is trying to the save largest inland lake in Ohio. It's a dying lake thanks to algae and farm run-off, and it's killing the local economy. It's going to cost a lot of money to fix the lake - millions of dollars.
I'm just curious what the donations go towards or what solution is proposed? It seems the problem is with runoff from high nitrogen chemical fertilizers causing algae blooms? In water ways I'm familiar with that had algae blooms, a public private partnership focusing attention on existing regulations managed to get waste products from livestock manged better and dramatically reduced the blooms and the harm that came from them. It didn't need a lot of money, it just needed people to pay attention to the right things, and some agri business to put out some costs to change to comply with existing laws. Obviously that isn't true everywhere, I'm just wondering what your situation is - is it to stop ongoing problems or to get it out?
There are several non-profit organizations that are collecting and spending funds for lake improvement, and there have been several studies done to try to figure out the best course of action. And the state has some funds directed at the effort.
My Bitcoin idea is to raise money and hand the money to one or two of the organizations that have studied the problem. I'm not pretending to be the expert on
the correct solution. Rather, I am trusting the people who have already spent thousands of dollars studying the problem. I would just act as the public figure of the Bitcoin community, handing over the giant symbolic Bitcoin to the organization. That would be cool.

Here's one of the organization's solutions page:
http://www.lakeimprovement.com/sites/default/files/edseries/solutions/solutions.html . As you can see, they have some solutions including both clean-up and prevention. Everything costs a lot of money, of course.
But I DO think the idea of trying to get a local community to rally around Bitcoins as a solution to THEIR problem would be pretty cool. There are a lot of local business people whose livelihood is threatened because of the dramatic drop in tourism.