Ko Samet, also known as Koh Samet, is an island off the eastern coast of the Thai province of Rayong. A popular weekend getaway for residents of nearby Bangkok, the entire island is zoned as a national park. It is more of a quiet place, and isn't famous for its parties likes some other Thai islands. Being that it is a national park, there is an entry fee. Supposedly it's 20 baht for Thais, and 200 for foreigners.
If you are looking for a place to stay, try to arrive early, and you'll have better selection. Also, cheaper bungalows usually get taken first. There are many places along the beach on the east coast. Make sure to see the room before booking, so you know what you're paying for.