If you want to park yourself on an island that is only in the beginning of development (nothing to do but sit on a beach), think about one of the other three other islands not far from Koh Chang. Koh Mak, Koh Kham and Koh Kood. Visit on a day tour or take a ferry directly from the Thai mainland.
Koh Mak has a population of only 500 and the entire island is only16 square kilometers. A sleepy place and low-lying. The method of getting around Koh Mak is on foot or by bicycle. The majority of the resorts are located in the bays to the northwest and southwest of the islands. There is good diving there and actually three more little islands off Koh Mak for day trips. A few Koh Mak links that I was able to dredge up are: Koh Mak Resort, Plubpla Koh Mak Retreat, Cinnamon Art Resort and Spa, and Koh Mak Holday. It wil be interesting to see how development of Koh Mak continues since 90% of the island is owned by one family.
Koh Kood is the second largest and southernmost island in the Koh Chang archipelago. A mountainous island covered with rain forest. Koh Kood population is 1,500 with one decent road but many of the resorts are only accessible by boat. You’ll have to hire a motorbike to get around and explore the island. Don’t expect much (or any) activity here other than sitting on a sand beach and watching the sea. Most of the resorts are luxury developments that cater to Thai package tours. Sorry, but information was very sparse.
You can reach both Koh Mak and Koh Kood either by local ferry from Dan Khao close to Trat that only leaves twice a week, or arrange transportation from Koh Chang. If “deserted” and solitude is what you are after now, give these two islands a chance before they turn into the next “in spot.”