Distances between the beaches are a little too long to walk comfortably, but getting around is easy, as the roads are wide and bike taxis (motodop) are everywhere.
The standard price is US$1 per trip, although expect to haggle at night or if the distance is long. They'll gladly pile on two people and their luggage too. For larger groups, car taxis can be called up by phone (flat US$5 to most places around town).
There are dozens of the ubiquitous tuk-tuks around the new bus station and the accommodation areas. They are some of the most persistent and over-charging drivers in Cambodia; they have formed an "association" for price fixing - a trip to the bus station, $2 in Phnom Penh, costs $3 if your bus company pays but they will hit you for $6 if you arrive at the bus station and want to go to the beach. If you have decided where you will stay it can help to call for a pick up; even if its not free, it may save you some overcharging.
The ride from the new bus station to Serendipity Beach should cost no more than $3 during the day.Another great choice to get around is to rent a scooter. Haggle a bit and you can get it for US$5 a day. Fuel is quite cheap and can be bought at many roadside shacks. However, as of 2009, renting scooters to foreigners is technically illegal, and the police may stop and fine you.