I also posted this challenge on LessWrong; but you might not want to click over there if you want to avoid spoilers/anchoring/etc.
How can we sustainably engender good in the world? Sometimes it helps to take an outside view, rather than being anchored in the specifics of your situation. One thought experiment I use for this is: what could I do with a time machine? To avoid crazy time-travel shenanigans (which are probably optimal if you really have a time machine), pretend your time machine is single-use-only. You can take a one-way trip to any time-period (possibly with some friends) and try to do as much good as possible.
Make the assumption that you can actually change the past (contrary to relativistic time-travel, in which there's just one self-consistent timeline).
All technological risks apply; so for example, if you believe in technological X-risk, then any technology which you bring back may accelerate progress, and may therefore give humankind less time as a whole.
I suggest using upvotes carefully, as judgments of which projects will work. If you want to give kudos for other reasons, such as insightfulness, leave a comment.