The best way to experience the Similan Islands of the west coast of Thailand is via a liveaboard dive boat. Spend 3-5 days doing nothing but eating, soaking up the sun and diving. This is the best way to get in all the really good dive sites as well as some of the land attractions, and its not even that expensive.
Tip: Avoid the week before and after Christmas, and don't book online, instead head to Khao Lak (the jumping off point for most boats, just north of Phuket) and ask around. Go from dive shop to dive shop looking for the best prices and you are sure to get some killer deals!