Kbal Chhay Waterfall is the main inland nature excursion from Sihanoukville, reached by heading north from town and then along a rougher access road. The falls are modest rather than dramatic, but the wooded setting and swimming pools make them popular with Cambodian families.
Go with the right expectations. Food stalls and shelters line the area, so the experience is more local picnic ground than untouched jungle. It is best in or just after the wet season, when the water has enough force to justify the trip.