Samui has an excellent public transport network of songthaews - basically, converted pick-up trucks with bench seats. Dozens of them circle the island on various fixed routes all day and fares start at around 20 baht for a short trip - this is paid at the end of the journey. There are no fixed stops, so flag one down anywhere and ring the bell or bang on the roof to get off. After dark, many songthaews operate as private taxis, so it's necessary to negotiate a fare before getting aboard. Expect to pay several hundred baht to travel between beaches.