The Tonle Sap lake is a good excursion from Siem Reap. We went in late December 2014. I carried my Sony A7R as well as the Contax G2. The Contax was loaded with Kodak Portra 400, which I rated at ISO 100. Now, that’s a tip for any newbie to film. Overexpose by 2 stops or so because color film (not slide film!) as a lot of latitude in the highlights but very little in the shadows. So just overexpose and then pull back the shadows in Lightroom or similar software.
One fascinating attraction connected to the rivers that drain into the Tonle Sap is the Floating (or Flooded) Forest. These are flooded mangrove forests surrounding the cluster of Kompong Phluk villages. You visit them on small boats rowed by local villagers. These small boats are owned by separate families and is often their only source of income.