Probably a bigger time commitment than implied, but the UN [0] and code for america [1] list volunteer opportunities.
[0] https://www.onlinevolunteering.org/en/opportunities?f[0]=fie...
[1] https://brigade.codeforamerica.org/?_ga=2.40007591.405176958...
Help users of nonprofits understand them better - watsi, Kiva etc share data that can be visualized in a variety of ways.
Teach (blog, video etc) - this is especially effective in topics you already know, but have gaps. Teaching forces you to clear those gaps.
Do exercises from project Euler, Rosetta etc