The small settlement of Ban Salak Khok is a great place to visit by itself and for good food and relaxed atmosphere - well worth the outing from the west coast.
However, the key thing to do here is to paddle, row, boat among the mangroves. This you can do on the cheap or get the full set with food and drinks served out in the mangroves. If you have not seen mangroves close up before or understood the mangrove/mud flat tidal ecology, this is your chance to learn something.
The main restaurant her serves real Thai food, not modified for the ketchup crowd. And local raw materials. It is an interesting community out here, houses on stilts, connected with board walks.
Directions: South West coast of Koh Chang, in a bay dense with mangroves and sourrounded by green hills.