From Siem Reap, you can explore Tonle Sap lake and the floating villages. Before you book a tour though, I advise you to read some of the tripadvisor reviews to adjust your expectations.
One of the trips I took while in Cambodia was an organized tour to the Kulen Mountain, the river of the 1000 Lingas and nearby waterfalls.
When you want to explore Cambodia, there is no way around the temples of Angkor. The city that sits right next to this UNESCO world heritage site is Siem Reap. And its airport is conveniently connected to many cities in Southeast Asia.