Visit an Orphanage: There are few if any ‘straight’ orphanages in Phnom Penh: almost all are scams and do not have the best interest of the children at heart. Try contacting organisations that run educational programs and see if there is any way you can assist.
CHOICE: A great way to help the needy is to help the expat run charity called CHOICE. They help provide food and basic supplies to families and also provide vocational training and send many children to local schools.http://www.choice-cambodia.org
Mekong Cruises: Sunset Cruises leave every evening with many providing snacks or dinner. Be sure to visit Mekong Island to see rural life.