If you want to discover Koh Chang and its surroundings with an experimented, reliable and qualified guide, then I recommend you Teerasak Suamcheepmasau, also called Thomas from Thomas Koch Travel & Adventure.
Thomas knows all the beautiful and interesting places of Koh Chang but also of the Kingdom of Thailand.
He is a living encyclopaedia, he knows the names of each flower, plant, animal, monument, food, places, in brief he knows everything, he is a life style teller. He is very friendly if you have any request he will do his best to please you and make the trip and the visits as pleasant as possible.
Thomas is independent and with his minibus he can drive you anywhere you want. He can also organise private transfers from Trat airport to your Resort in Koh Chang or vice versa.
Thanks to Thomas you can discover the north and east parts of Koh Chang which are not as popular as the west coast. In one day he will drive you to the different sites and give you all the explanations you need.
If you have more time to spend, Thomas can arrange other trips away from Koh Chang. He can bring you to latex plantations, temples, fishermen villages, local markets, farms, but also further and show you the real Thai life.
2 places I would like to recommend: Suncity View in Lonely Beach and Baum's Restaurant & Butchers Shop in Klong Prao.