If I suddenly had a lot of money, the first thing I'd do is build a proper community health clinic in my home village in Sindh, Pakistan. I'm a twenty-seven-year-old software developer in Karachi, but my parents and many relatives still live there.
The people who'd benefit are the families in our village and the dozen or so smaller settlements around it, probably several thousand people. At the moment, the nearest decent hospital is over two hours away by road, and the road floods during the monsoon. Pregnant women, elderly people and children with fevers are the ones who suffer most.
I'd organise it carefully, because I've seen projects fail. I wouldn't just put up a building. First, I'd talk to local elders and women's groups to understand what they actually need. Then I'd hire a qualified doctor and nurse, offering them good salaries and housing so they'd actually stay. I'd also set aside money in a fund so the clinic could keep running for at least ten years, and I'd train a few local young people as health workers.
I'd choose this because of my grandmother. When I was eleven, she had a sudden infection, and by the time we got her to the hospital in the city, it was too late. The doctor said that if she'd been treated a day earlier, she'd probably have recovered. That has stayed with me ever since. Of course I'd also buy my parents a nicer house and maybe travel a little. But a clinic is something that would outlast me, and it would mean no other family has to go through what we did.