You should probably spend some of your money if you are a billionaire, bunkers and islands seem like a good buy to me (as well as farm land or just large swaths of land in general which other articles have speculated Bill Gates and others are doing for nefarious reasons). Bunkers aren't a rare resource but there's not many existing ones now and they can be difficult to construct in some countries for code reasons or take a while, so seems reasonable to buy an existing one or start construction on one if you have the cash. Farmland and islands are a bit more obvious of a great investment, they are finite resources, always useful, and can be used to generate income.