Skip touristy Sihanoukville for tranquil Koh Tonsay (Rabbit Island). Boats from Kep leave every day.
Rent a $5 a night bungalow on the beach for a few days, chill out or spend a few hours walking around the island. For lunch, try the locally caught crab in pepper sauce (Kep nearby is famous for its pepper). For $4 a plate, you can't go wrong.