Koh Chang has something to offer for everyone. Some of the activities are: serious sun tanning, snorkeling, diving, kayaking, mountain biking, tree top park, hiking, elephant trekking, fishing, swimming, waterfalls ... and of course relaxing on one of the many great beaches, tasting the fresh seafood, or even learning how to cook it!
Most development is along the west coast, beaches from North to South are:
Hat Sai Khao (White Sand beach), beautiful white sand beach with lots of hotels and restaurants along the road. It is the most developed area of Koh Chang, although the beach never has that many people on it! Nice and easy to stay here, everything close at hand.
Klong Prao, offers more secluded hotels on a nice sandy beach, most of them quite isolated but offering a great setting, peace and quiet.
Kai Bae, more of a family beach, has some nice hotels on the beach and guesthouses along the road. On the main road you'll find shops, restaurants and bars. Central to everything and easy to get around.
Hat Ta Nam (Lonely Beach) is the backpackers beach, many bungalows and guesthouses and a few hotels. There is something for everyone here, if you like a party you will find it, if you prefer the quiet then head South of lonely beach and you will find your spot.
Bailan is the new area to head to if you are looking for peace and quiet, many small guesthouses and bungalows on a pebble beach.
Bang Bao village is the southern end of Koh Chang, from here all the boats leave in the morning for transfer, diving or snorkeling. There are some great seafood restaurants and many small shops. In the evening Bang Bao is very quiet, making it also a nice place to stay if you look for a simple Thai lifestyle.
On the East coast there is some development, and quite a few things to do. Since the main road is not going around the island, it can be a bit tricky to get there, unless you drive the motorbike, rent a car or a taxi. It is worht the effort though, since the mangroves, bays and views on this side are absolutely stunning.
There are a few nice guesthouses not too far from the ferry stop, and some backpackers places all the way to the Southern point. Great places to visit are the many waterfalls, Salak Khok fishing village, Salak Phet fishing village and restaurants, Koh Chang memorial, ...