Talk politics.

Voluntourism is a scourge. If you don't have skills that the locals don't have, why are you there? You can earn money in the first world and send it, that's worth much more than your labour. Having said that I absolutely understand the desire to combine travel with helping people, but you shouldn't be putting people out of a job (like construction). There is actually a large demand for people interested in working with animals and the disabled in the third world because the society cannot afford to look after them themselves. Volunteering at a dog shelter in Thailand can actually bring a benefit to the local society. I know one western NGO that is trying to get the wild dogs under control on an island in Thailand. The dogs aren't native, so they're upsetting the local ecosystem and they are dangerous for the locals to boot. Without these Westerners nothing would be being done about the problem, so you can say that there is a genuine need and benefit there.