This Spiti conservation-and-development NGO was closely involved in setting up the Kaza area's rural homestay and trekking program and is a fine contact for organising anything beyond a casual visit to the area's villages. Ask about its mountain-bike/culture/wildlife tours, cooking/pottery/yak-rope-twining classes, and 'nun for a day' experience in Spiti's Buddhist convents.
Ecosphere has a variety of volunteering opportunities. Note that Lonely Planet cannot vouch for any organisation that we do not work with directly. Travellers should always investigate the standards and suitability of any volunteer project themselves.