Spay and neuter clinics across the country. My friend and I were actually planning all this out if I ever won the lottery (he knew I’d buy one sports car and a house, and then put the rest into a private foundation) and for the maximum impact, this seemed best. We’d work to get vets on board to do it at cost, and then the foundation would pay for it.
Then there was his crack elite squadron of porcupine rescuers (but I think he was kidding on that one. I think.)