The major reason why people come to Samui is to enjoy the beaches. Even though the two main beaches of Chaweng and Lamai have generally suffered due to mass development over the past decade, they are still relatively impressive.
As on many islands in Thailand, motorbikes are available for rent. Hire a motorbike, electric bike, bicycle or a car to get around. A day or two is enough to see whatever is to see. But remember, its entirely a beach destination.