I'm willing to admit that I would allow myself a few things that I'm not apt to have in reality. A garage large enough to hold a couple of special-occasion cars. I might start with a '57 TBird, and (here goes maybe a couple of hundred thou) a '56 or '57 Continental Mark II, with drive train and suspension from a Lexus 460. Once in a while, if another classic tempted me, I might let one car go and replace it. The other "guilty pleasure" would be travel, to a whole lot of places; even with the billion, neither my wife nor I have any real inclination to travel in highly expensive style; we just like to see stuff.
Next priority: be sure the young people in our families have the opportunity for whatever education they desire. Some of them are set for that already, some not so much so.
Salt away enough to be sure we would always be comfortable, and that leaves by far the most of the billion, and a lot of decisions as how to do the most real good with it. We know people who were largely responsible for starting an orphanage in Baja, Mexico. Besides the orphanage itself, they provide food, clothing, and shelter assistance for a lot of deperately poor working families who are brought into the area from further down Latin America. Beyond that, who knows.