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.