One of the best local projects I found in Central America.

I'm an eco engineer and was very happy with the treatment received in Quixaya by Cepek. I have travelled all over the world building things like vertical wind turbines, Rocket Mass heaters and solar energy absorbing installations. in Quixaya I was mostly busy building a hydro-electric turbine and before that on different sites in Guatemala I taught at different projects how to build a variety of appropriate technology installations. Quixaya is very recommendable particularly because of its permaculture operation and it also offers a variety of recreational activities. It is very safe and volunteers can expect healthy and well-balanced meals.
If you're wanting to help out on a well run by-locals-for-locals highly sustainable and well minded community agriculture and social project, you'd be hard pressed to find one better than this.