If you are joining the travel agent from HK, apart from some arranged visits and shopping, they will advise you on some local tours to the islands, mangrove, etc. So you don't need to plan so much. If you are going there by yourself, you may take a look at the following sites to get some ideas:
Actually I think Langkawi is a bit dull and backward in comparison with those popular resort areas in SE Asia like Phuket, Sabah or Pataya. Just enjoy the sun and relax yourself.