You get to keep only enough to maintain a very modest lifestyle in a low-cost-of-living area, the rest of it has to go towards improving the world in some way.
Edit: Given the previous rules that you must maintain a very modest lifestyle in a low-cost-of-living area, would you rather choose to opt out and not have the money at all?
I would build housing for the homeless. Simple rule, no muss no fuss: need a place to live? Get a place to live. Live there for free until you can get clean and/or find a job and get back on your feet. Maybe throw in some job training too, or some kind of work-study program where they can get (paid, ofc) experience while they learn a new trade to help cover gaps in their work history and such. I’m imagining apartment complexes built around some kind of combination trade school/recovery program that teaches people to be plumbers, welders, electricians, etc while helping to get and keep them clean, offer group support for reintegrating into society, the full package.
I could help a lot of people that way with $5 billion. I’d show up in places like NY/LA with large homeless populations with a greyhound bus that said ‘free apartment and a good job this way’ on the side or something too, and just bus 'em in as new housing became available.
I wanted to do something similar to your apartment idea but for teens and young adults in poverty-stricken areas. Get them into a stable environment, teach them the necessary life skills like credit management, budgeting, cooking for yourself, etc. and give them job training. All of that would also go just as well for the homeless population. I grew up in an area that was riddled by the crack epidemic and had no resources for kids/teenagers to do anything other than crimes. The city didn’t pay for anything to keep us out of trouble, and our drug addict parents sure weren’t going to do it either. So I always felt like if I had a ton of money I would focus on program to help teens in those situations learn to be proper adults, because nobody else is teaching them. Schools won’t, parents won’t, we’re defunding every program that helps outside of the home. It’s a mess.
That sounds pretty good too, yeah. The program could be expanded to include safe spaces/training for teenagers and such too, probably include a rec center that always has stuff going on for them to participate in, etc. At that point it turns from a housing/job training project to basically a community, built around safe spaces, opportunities, and education for the future.