Care for elephants and learn an enormous amount about these gentle giants. Prepare their meals, feed, walk and watch them socialize and bathe.
Paraplegics have a tough life anywhere, but it is worse in Bali as they are treated as outcasts. You can imbue a paraplegic with autonomy and dignity and provide him/her with a new lease of life.
You can change the life of a needy Cambodian family by helping build a decent place for them to live.