I'd suggest doing what so many ultra rich people do, just sit on it so I can feel rich, really that's more 'frivolous' that actually spending it and sending the money back into the system. Honestly, if you've got 10 million in the bank, it's probably safe to go wild and enjoy spending some of the rest. If you've got 50 Billion, hell, hire a few thousand people to just wear silly hats and salute you whenever you walk past. We NEED some frivolity from these people so money actually moves, it's like an anorexic who owns a food store locking their doors to everyone else during a famine, just in case they decide they want to eat LOADS later on.
I don't pretend to be an expert, but I do lay a fair bit of the blame for the current financial situation at the feet of the mega rich. Money hasn't disappeared, it's just gone from regular people and governments to the 1%, who, if they actually stopped hoarding it and tried enjoying life, instead of just sitting on it smugly being rich, might actually enable that 'trickle down economics' thing to happen.
In the UK, our national debt is near 1 trillion pounds, yet the 1000 richest people made nearly half that only last year.
Forgive me for being all serious in a fun thread, but it grinds my gears to see everyone having to lose things, for libraries and hospitals to close, just to jam a few more banknotes into rich people's mattresses.