Traveling in Cambodia for 3 weeks over Christmas. Going from North toward south.
In Phnom Penh we signed up for an overnight tour with a company called Betelnut Tours. http://betelnuttours.com
It was run by a Cambodian & Dutch couple living in Phnom Penh and, although on the pricy end of Cambodia experiences, we had a fabulous time. We got taken to some of their favourite local picnic spots outside of Phnom Penh and spent the night at their family farm cooking together in the outdoor kitchen and hanging out in the hammocks just chatting. I think once you have a look at their website you'll get a really good feel for the sort of people they are.
My second authentic experience recommendation would be to find out a bit more about some of the social enterprises running in Siem Reap.
We discovered these by accident by picking up a brochure from ConCERT http://www.concertcambodia.org and learnt so much about the challenges facing the Cambodian people by just asking questions when we visited these local businesses. Included in this recommendation is must do visit to the incredible GreenGecko project http://www.greengeckoproject.org
It's also worth looking over the ConCERT website to read up on how to be socially responsible tourist http://www.thinkchildsafe.org/thinkbeforevisiting/ because it seems there is whole heap of victim tourism in Cambodia branding itself as authentic inside experiences. over 3 years ago