Koh Chang is great
If you'd like to party, then stay at White Sand Beach, if you want to relax, I would recommend you Khlong Phrao, if you're looking for something in between, stay at Kai Bae or Lonely Beach.
Also I would recommend you to take "5 island trip" in any agency - I'm sure you won't regret it.
From Koh Chang you can easily get to the nearest islands - Koh Mak & Koh Kood by boat. They are much less developed than Koh Chang though...
Between these two I would definitely stay at Koh Kood - it's green, has great beaches with a clean water and there are not so many tourists.
Have Fun & Good Luck