Here’s a very nice travelogue, a 7 minute video shot recently of the delights of Koh Mak. If this doesn’t convince you to spend some of your holiday time on the island, then nothing will.
Koh Mak is about 45 – 60 mins by boat from Koh Chang and also easily accessible from the mainland. it’s possible to do a day trip there – but you’d only have , at most a couple of hours on the island, due to the boat schedules. But ideal for ashort break from koh Chang – especially if you are looking for peace and quiet and an island you can bicycle around safely. ( No traffic and no steep hills.)