On Kai Bae beach you can rent kayaks and take a little swim in the small neighboring islands. They can be quite complicated. Kayaks on the southern part of the beach, though there are cobbles in the water.
After this beach there will be a sharp climb uphill and a beautiful observation deck where you can perfectly see the sunset. I have come there many times and advise you. And if you drive along the highway a little further, then at the top there will be the sharpest turn on Koh Chang and the road to Siam Bay Resort, which has its own private private beach. The place is very quiet, with good prices, rooms start at 700 baht and above. Many bungalows have sea views and they stand on a hill, generally very beautiful there.