Ko Chang (big)

photo by panoramio user Reza1001

Second biggest island in Thailand about 300km away from Bangkok and largest island of the Ko Chang Marine Park archipelago.

Wikitravel says:

Ko Chang is one of Thailand’s most beautiful islands with long white sandy beaches, most half deserted. The island is also home to a wide range of wildlife, including a good selection of birds, snakes, deer and a number of elephants. The island and its vicinity are great places for snorkeling, diving and jungle hiking. The “discovery” of the island as a tourist destination since 2000 has brought on a large amount of rapid development, and while still far quieter than places like Phuket or Ko Samui, it’s probably better to go now than later.
Ko Chang has the same seasons as the capital Bangkok. The best season to go is the (comparatively) cool season between November and February. March to May are roasting hot and between June and October it rains, and a lot at that: 4,000 mm in an average year.

Wikipedia says:
Ko Chang (Thai: เกาะช้าง), also Koh Chang is the second largest island of Thailand (largest island in the Ko Chang Marine Park archipelago), located on the Thai east coast 310 km away from Bangkok near the border to Cambodia in the Gulf of Thailand. The name means Elephant Island. Ko Chang was named for the elephant shape of its headland, although elephants are not indigenous to the island. At present, there are 8 villages in total. It is a mountainous island and Khao Salak Phet is the highest peak of all at 744 metres. ... read more