If you like nature, don't miss the Botum Sakor national park, situated in the Cardamom mountains in the south-west of Cambodia. It's a haven of peace that is still protected from deforestation, where you can admire the diverse flora and fauna of the country.
Phnom Penh region was where most of the atrocities of the Khmer Rouge dictatorship were carried out. Nowadays there a numerous places where you can learn more about the history of the country and you really should visit them during your trip to Cambodia if you want to learn more about the challenges faced by today's Khmer culture.