Koh Samui is located in the "Gulf of Thailand". Although the sea here is not as clear as in the Andaman Sea, the beaches on Koh Samui really amazing. Both Lamai Beach and Chaweng offers a very comfortable and relaxing experience without all the vendors that Phuket is famous for. In the north of the island will find such Maenam beach, often used by the island's local Thai population (which says something about how good it is, of course).
Koh Samui's nightlife is mainly in Chaweng. Lamai has also a fairly active nightlife. Koh Samui is not the same huge range of party places like Pattaya and Phuket, but there is still a place for everyone. Unlike other places in Thailand that offers Chaweng an impressive selection of restaurants with very high quality that serves food from around the world and then not only from the Asian cuisine.
The roads on Koh Samui is not very good and the terrain is at times very steep. Koh Samui is also famous for the ugliest accident statistics for motorcycle accidents in Thailand, so think carefully before you rent your car in Koh Samui.
The friendly islands of Koh Tao and Koh Phangan can be reached via ferry from Koh Samui.
Elephant riding are several places on the island. These are part of the authorities' attempts to move the elephants who have traditionally worked in Thailand for many forests that are now protected. The need to help elephants in Thailand is very big, so by visiting and riding elephants helps you automatically elephants themselves.
Besides elephants, there is a good variety of different fun activities. The island also has its own zoo and a lot of beach activities there.
Chaweng has some stores, but things cost more here than anywhere else in Thailand, so be prepared to spend some money if you go to the more crowded tourist spots on Koh Samui.