I'm going to cheat here, I'm going to say "I could plant trees all over the world."
American Forests has a program called Global ReLeaf where for every dollar you donate, they will plant a tree. Trees are planted in urban areas as well as in less urban areas. Attention is payed to restoring natural habitats and ecosystems.
Here's a list of all the places you can plant trees with American Forests.
$600 = 600 trees all over the worldWant to get some more bang for your buck?
Think that the trees must be going away for some reason?
Maybe it's all that junk mail you keep getting.
There are a number of companies that promise to reduce your junk mail. My wife got Green Dimes to do it for us. For $20, they will stop your junk mail and plant 10 trees using one of their affilliates.
$20 = A lot less junk mail + 10 treesYou might want to take note that there's a very interesting article in Portfolio.com about the business model of Green Dimes and its parent company Tonic.