On the west side of Koh Rong is Long Beach. It is an empty and fairly un spoilt area of the Island. The beach itself is probably the best area on Koh Rong to catch the sun set.
To get to long beach you need to hike for one hour across the island through a jungle. There is a small bit of climbing involved but it is worth it as the beach is quite spectacular.
If you are feeling lazy though you have an option to squeeze into an over filled water taxi that will take one hour and cost roughly five dollars.