A year too late for this response, but nonetheless: OP, I'm not clear whether you wanted to volunteer in order to make a difference/give back, or merely to improve your chances of being granted time off.

Most volunteer opportunities are more or less useless in terms of providing value to the locality. Many are destructive. I'm not going to go on a diatribe here, or even to try to defend that position: if you're interested, you can easily look into it yourself.

Best bets for providing something of actual value to people other than yourself will depend on the skill set you bring. Do you have civil engineering experience, professionally or otherwise? How about machine repair or fabrication in the absence of fancy tools? Small-scale farming? Are you, perhaps, experienced in the legal aspects of immigration or refugee processes? Do you speak the language(s), e.g., Spanish, Portuguese, the lesser colonial languages (French or German, depending on where you go) or indigenous languages?

In the absence of specific, useful skills, the best way to effect positive results is likely to involve spending a bit more of your valuable time earning money at home-country wages and donating it to direct service organizations. Otherwise, just look for a feel-good experience which will enrich your life and travels...but try not to interfere too much or to leave a path of cultural destruction in your wake. For example, short-term volunteering in orphanages is generally considered bad news; helping build schools maybe slightly better, though still highly suspect; wildlife sanctuaries full of fee-heavy scams.

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Hope that's helpful.

Mark