Me? As much as I would love to spend that money on the Imperial Sand Dunes Recreation Area, I would spend it somewhere that would benefit much more... AND probably make me some money in the long run.
I would try to help out the Salton Sea. I would first buy up as much property surrounding the Salton Sea as I could (for future profits). Then I would build a desalinization plant as big as I could possibly afford to help rid the Salton Sea of it's salt problem. Then I would hire specialists to see what other problems I could help fix, such as lack of oxygen in the water that kills millions of fish. Perhaps an oxygen pumping plant would work? Whatever it took, I would make the Salton Sea a place where fish and birds would thrive (the birds already do, but millions of fish die because environmentalists won't cure the problems with the Salton Sea). Then, once it was not such a stinky, smelly, salty body of water anymore, I would make back my billions by selling off all my property to the avid boaters, personal watercraft riders, and fishers that would flock to that place twice as much as the amount that go to Havasu. So I helped the fish survive, which helps the birds that eat fish thrive, AND I helped humans by providing another place for them to recreate.
I left some of it out, like all the bars, docks, campgrounds, and such that I would build too.
What would you do?