Is there one or two things you prefer to do more than others? There are loads of volunteer opportunities to choose from. It'll make it easier if you can narrow it down to a preference.....
Anyway, check out Transitions Abroad for a good list of some of the opportunities available. They are a stand-up publication and there should be some great places taking volunteers.
Check it out here.Personally, I think it would be really cool to volunteer as a nature guide

I've heard Common Hope-helping poor families in health, education etc-is excellent too but you have to be near-fluent in Spanish. I'm sure for some of the other positions you do too. Most volunteer placements do cost some money but it shouldn't be much more than covering your own overhead (food, lodging etc). Be sure you know exactly where your money is going. Some places
are crazy-expensive.....
It's kind of you to want to offer your time to help in some way. That's a fantastic way to give back some while you're on the road. Plus, it'll offer alot more insight into the culture and people as well as the region you'll hunker down in. The benefits are plenty! Let us know what you're thinking, or if you choose something. I'm interested to know what you pick in the end!