Ko Samet is an island that is only a few hours drive from Bangkok. It is very popular with the Bangkok crowd, and it can be difficult to find a room on weekends. It is part of a national park, so you have to pay the usual 200 baht, in additionto whatever you hve to pay to take the ferry out to the island.
This is the dock with the "ferries". You really don't have many options for getting out here, and it doesn't seem expensive, but all the little charges (bus, ferry, taxi, park fee, hotel) add up. It pays to call ahead to book a room, rather than wander around aimlessly like I do. You can also book rooms on the mainland.