Anywhere to volunteer for a week or two during my journey in Cambodia/Laos?

Going to Cambodia and Laos for a summer journey but would like to spend a week or so volunteering along the way. Looking for a place that I can just walk up to rather than signing up and all that stuff. I would prefer something related to medicine or health care although I am open to any sort of volunteer work (e.g. orphanage, conservation, etc.). Any suggestions would be great! Thank you