The Kbal Chhay Waterfalls, approximately 16km (9km on national road #4, turn left and 7km more on gravel track) from downtown Sihanoukville, is a great spot to unwind and revitalize while catching the fresh water of the 14m high fall right on your back. The waterfalls were discovered in 1960 and developed into a freshwater source for Sihanoukville in 1998. A short ride from town and a great to spend your day lazing around.
There are several food/bbq/fruit/snack/drink vendors with reasonable prices so you do not necessarily need to bring a package lunch. This lush area gets very busy with friendly locals during weekends and holidays.