Wow, last thing I figured is my answer would be given by someone else. I love the negative results answer.

I'd also:

Endow a non-partisan original-reporting only, no-pr, news site with no sponsors to bias results, ever. Now that all news is corporate sponsored in various ways, we need alternatives.

Use part to endow a rationalist organization dedicated entirely to the spreading of rationality as determined by the results of research on how to most effectively spread rationality.

Create a cost-effective ethically acceptable platform for testing drugs and theories on a human-ish system. Basically this would be an attempt to strap a rocket to modern day testing and drug discovery by making it as effortless as it would be if you could put humans in various conditions without ethics or harm considerations.

See if there's a way to use my money to undo the corrupting supreme court Citizens United decisions and decisions about Super PACs.