The thing I keep coming back to again and again is public housing. I would just start buying up land and building public housing. Getting the homeless off the streets would improve not just their lives, but also the lives of the people who already have homes who happen to live in areas with a lot of homeless.
I've also considered public pay-what-you-want restaurants.
If anyone wants to give me untold billions, I will get right on it.
[0] I would totally go for a private train car, though.