For WaterAid - various volunteer opportunities available, from event help and awareness-raising to part-time jobs.
Go to India! with Children Walking Tall, a charity providing sanitation, nutrition, education and medical care for street children.
Or maybe Latin America. Preserve rainforests or coral reefs, conserve turtles, study dolphins or help people build toilets....
Too far? Go to Mid Wales for a week and help out at the Centre for Alternative Technology.
Even Machynlleth is too far? Never mind. There are loads of conservation and environmental volunteer opportunities in England.