pick a resort on any island you can’t go wrong.the tourism there is endless, i spent six months and only marked the surface never mind scratched. keep an eye on the weather around october , is this season change time?
i wouldn’t even look about biking personally but bali has a good scene as does philippines but way too far from hong kong. the heat and sun , humidity and mozzies, remoteness would put me off biking.
trip advisor and lonely planet are a good guide. id recommend a spa resort in cambodia , see angkor wat. do the beach tourist stuff. its cheap to do , but still very poor and 3rd worldly so be prepared for the culture shock, though if your hong kong based you know the story.