On the last day of your trip, explore the Koh Rong Island on foot and spend some time relaxing at the beaches. Try different cafes and food stalls here and party on the beaches for an epic time.
Next morning, leave for Koh Rong island on a fast ferry which will take 30 minutes to reach. You can choose a boat tour which will include Fishing, Snorkeling, Island hopping, BBQ, Village tour, etc.