Easy. Id go up to a billionaire and buy a billion of his money and just keep that.
Or for a "real" answer Id buy a billion dollars worth of real estate. The first thing I would do would be buy all the houses that have been on Real World, then with the money left over Id buy places in all the "posh" parts of the large metropolitan areas of the world.
Or Id buy a major sports team. Im thinking either a NBA team or an English or Italian soccer team.