Langkawi is a beautiful place with nearly 100 islands machan. you can do island hopping, eagle feeding, snorkeling etc....
if u go by bus it will take nearly 6hrs from KL, so better take night bus. Then from the jetty u can take ferry ride around 45mins.
@ their u have few places to see, eagle square, duty free, cabel car, water fall... etc.... go and enjoy... i have been there few times ...