Koh Chang in Trat province is the last of Thailand's big islands to be developed into the "next Phuket". It's the "last" quite literally, as the island sits just off the coast of Cambodia. Koh Chang is about as far east as you can go along the coast and still be in Thailand.
There is a nice selection of resorts on the island, from budget to quite posh. The main drawback to Koh Chang has generally been the difficulty getting there. With no airport, the only way to get there has been a long drive from Bangkok. But now boutique airline Bangkok Airways has revived Trat's airport and now operates two flights daily between Bangkok and the provincial capital. From there, it's about a 20 minute ride to the pier for the ferry to Koh Chang.
The fastest way to get to Koh Chang is to take a Bangkok Airways flight to Trat province, then transfer to the pier in Laem Ngop for the boat to Koh Chang. At last check, there were two flights a day between Bangkok and Trat. The pier is only about 20 minutes from the airport, and transfers are available from Bangkok Airways directly. The cost is 250 Baht per person and vans run every 30 minutes from 6:30 to 18:30.
The longer, cheaper route is by bus to Trat from Bangkok's Eastern Bus Terminal on Sukhumvit Road. The trip takes at least six hours, so it's an all-day jaunt to get to Koh Chang by bus. If taking the bus, on reaching Trat you will need to catch a songtaew to the pier at Laem Ngop. There are boats to Koh Chang about every hour during the high season from November to May.
The island sports a wide range of resorts and bungalows to stay in. There is a good selection of up-scale resorts, since it was the intention to promote the island as a premium destination. However, there are a fair number of budget accommodations as well.