Just find expensive things you can spend the money on. So, you've got enough money to buy a planet. Problem is, the only planets that have titles available all kind of suck in their own way, so they're fixer-uppers that need major boosts to their education systems, infrastructure, and economic bases... then if you build those, pirates want to steal it, because they're far from the front lines and underdefended, so you have to hire mercenaries or establish your own defense forces...
So say you navigate all of this and become the billionaire playboy philanthropist ruler of the Jewel of the Periphery. Then what? Then you face the named pirate bands in the books, whichever enemy house or periphery state you're nearest starts raiding your world, the expanded economy makes industrial espionage and sabotage into real issues as you suddenly butt in on long-established trade patterns, and the established (and higher) nobility of the realm becomes resentful of the new upstart that's making them look like a bunch of leeches that live the high life and contribute nothing to society and start making plans to bump him off, take what he has, destroy his reputation, or any combination of the above.
Rich people have problems too, and those can scale up almost indefinitely. You think Maximilian Liao doesn't have problems? How about the First Lord of the Star League? Oh yeah, they do, and not just the ones inside their own heads.