Koh Rong Island is the biggest Cambodian resort island off the coast of SihanoukVille. 

Beachside bungalows around the beaches, and virgin forest, hills, and waterfalls inside.  It's currently 99% undeveloped, however, plans for a massive resort complex, complete with golf courses, and airport, casinos, and the works are taking shape.

Right now, you can visit the island on a day trip, or stay in one of the few beach side bungalows.

Most of the island is unexplored, and covered by dense jungle.  There are several smaller islands surrounding Koh Rong, and you can visit them in a quick boat trip.  Scuba diving, snorkeling, swimming, exploring, and fishing covers most of the activities available on the island.

The trip to Koh Rong takes 2 or more hours from the mainland, in SihanoukVille, Cambodia.  There are a few speed boats around which can get you there in a half hour however.

Also, another completely undeveloped island, with the confusing name Koh Rong Samloem, lies just 4 Kilometers South of Koh Rong Island.