Getting around Samui is very easy to do with local taxis, the so-called "soongtaos", which are covered pick-up trucks. They are the main public transportation means. The fare normally depends on  the distance. It's recommended agreeing on the price before getting in.

But if you prefer to be free and independent to discover by yourself the many hidden places on Koh Samui, the ideal is of course to rent a vehicle.