i stayed in the Amari Coral Beach resort, great private beach, pier with lots of coral to see while snorkling, can hire the gear in the resort, very cheap, walking distance to patong beach for shopping or entertainment, great spa/massage as well, individual huts facing the beach, great for couples, can hear the waves comming in, great atmosphere at night to have a massage.
If i were u, i would find another 5 start resort to stay as the banyan tree is not cheap, i always think its over rated. even the banyan tree in bangkok, when i heard the price to stay there, i wondered what is so special and what extra service do i get for the extra 1-2K HK dollors.
go to any travel agent and find a good package deal and try to extend a few nights. my package deal to phuket cost me 3K per person sharing for 4 nights, fly by thai airways and airport pickup/drop off. great deal.
this website lists all the travel agents in HK, pick one that looks good to you.