If you don't want to spend money and if you feel lazy to do anything, this activity is made for you. It can be done anywhere, on the beach, at the swimming pool, in a Spa, wherever you want.

If you are planning to chill on the beach, remember to use sun protection. Even if it's cloudy you can get terrible sunburns which can spoil a part of your holidays.
Reading on the beach or just taking the sun, watching the sea or playing pétanque, find your own way to relax and to forget your stress.

The best relaxation you can get in Koh Chang is with a massage. If you don't fall asleep, for sure you will be close to Heaven. There are massage places everywhere on the island, but few of them are made by professionals. 
The most famous is the Thai massage, rough and painful, but some people like it a lot. Oil massages are softer and very relaxing. Coconut oil and Aloe vera gel massages are also very good for your skin, especially after long sun exposures.
If you are in White Sand beach, I recommend you Lagoon Thai Massage and if you are in Chai Chet, try the Spa from Koh Chang Paradise Resort & Spa.