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.
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.