I am involved with Dhamma based charity work in Cambodia in the form of printing of books etc and visit about once a year. Thre are many temples and monks but the standard of practice and Vinaya among monks is extremely poor. There are few temples, if any , that a foreigner could visit for Dhamma instruction and practice. (except perhaps for a Goenka centre outside Battambang).
There is quite structured instruction in the monastic schools and universities, but hardly any at the Wats except for the Dhamma talks after dana in the local language. In Phnom Penh, I know of only 2 possibilities - the Buddhist Institute near the Naga casino and Wat Langka that used to offer Mahasi type instruction in English some time ago. You may try to get in touch with Ven. Yous Hut Khemacaro the teacher, who will be the best person to give further leads.
There is more going on in terms of social work. In this respect I can make a few recommendations
1. Beth Goldring involved with care for HIV patients and hospital visits http://www.brahmavihara.cambodiaaidsproject.org/
2. Salvation Centre Cambodia - HIV care and social work http://www.scc.org.kh/
3. Buddhist Library healthcare work (mostly south of PP). This might be of particular interest as it is run by Aussies from Sydney. http://www.buddhistlibrary.org.au/proje ... nformation
4. Wat Opot - a community run by a Buddhist-Christian partnership to care for HIV children http://www.watopot.org/contact-information/