The closest airport to Koh Chang Island is in Bangkok, just over three hundred kilometers from the island. It's Suvarnabhumi Airport. Located to the east of the city, it is one of the largest airports in southeast Asia. It was built in 2006 and is still under construction.
The layout of Bangkok Airport isn't too simple, and you need to know the tricks. There's one terminal for international and domestic flights. The building has four floors. If you're flying in a group, don't use the self-check-in desks, as seats are randomly assigned and you won't be able to pick two adjacent seats when you check in consecutively.