More suitable for people who enjoy physical labour, building and maintaining, this type of work is a good option if you truly wish to help create more livable conditions. With positions generally available throughout developing countries, there are many different locations to choose from depending on preference and perhaps skills.
As with teaching, you generally will not be required to have any prior experience. There are many organisations that construct huts, schools, farms, playgrounds and even eco communities. While free positions are often available, many projects ask you to contribute towards materials used in constructions as well as petrol for the transportation of material.
Volunteering positions withing construction are some of the most flexible positions available, with the length of your stay often irrelevant (so long as you don't spend 1 week getting trained, then leave after 2 days, of course) and you can often opt for a couple of days out to go explore or simply recharge your batteries. There are also organisations where you simply phone the day before you want to work and they will arrange to pick you up the next morning, so as to allow you to combine your holiday with a bit of volunteering. It is worth bearing in mind that this type of work can be quite strenuous so expect physical work in hot conditions.