i've been to Cambodia twice. i recommend checking out the infamous S-21 prison tour:
http://en.wikipedia.org/wiki/Tuol_Sleng_Genocide_Museum as well, hiring someone to take you to a military base to shoot machine guns. quite the experience and it's a 10 minute drive on a tuk-tuk. if you are going to be there for more than a couple of days, i highly recommend taking a trip to Angkor Wat, the temples are incredible.