I actually have personal experience with this, so I might be able to bring some real-world insight here...

When I was at NASA in the early 1990's, my boss won the lottery. He took home somewhere just south of a million dollars. Mid-50's guy, married with two kids in Junior High School, IIRC.

He paid off the house.

He got a new - not flashy - car. Something comfortable and appropriate for the time/place/role in his life. It wasn't an actual Ford Taurus, but was just as boring (I can't remember exactly). But it was new, paid for, and comfortable for his 6-foot-3 frame.

He then put money away for the kid's college education.

Then he kept coming in to work. On time. Every day.

And it was AMAZING.

With all of his everyday monetary worries taken care of, and a sizable nest egg to support him in case of any emergency, he just relaxed and enjoyed himself.

And I gotta tell you, when your boss comes in every morning smiling, with no stress, and is able to keep his head above the everyday stresses of our daily lives? Suddenly YOUR life becomes amazing.

We still did all the same projects we always did at NASA - this was the Multimedia Design Group - but every project just glided along to completion. Every member of the team worked hard, but - for some reason - projects got reasonable timeframes. Collaborators enjoyed working with the team that was "so much fun, and so easy to work with".

So this virtuous cycle began, and continued through the duration of my employment there.

It was AMAZING.

So what would I do with a gajillion dollars? Basically become a Medici with a graphic design team and software development teams. I'd help non-profit organizations produce AMAZING marketing collateral and products. For free. I'd work with people who had great ideas, but couldn't get the dev team together to nail the idea down. For free.

A 3 story warehouse building with the amenities of a modern day software startup, but everybody in the building is working on projects they are CRAZY about, and have zero budget concerns.

That's what I would do.