We went two years ago for 9 days and loved it. There's loads to do - any decent guide book will have all the major sites in. I wouldn't recommend going to Aberdeen though, we thought it was a disgusting smelly place, and a complete rip off. Jumbo just isn't worth visiting (and one of my guide books had it in it's top ten - I would put it in the top ten places to avoid. Some of our favourite things were - The Peak (both day and night), the Peak Tram is worth using once just for the experience, but we also got the bus (a double decker bus on that route is a thrill ride in its self!), and a taxi (which for four of us was cheaper than the Peak Tram); the trams, they run along one side of Hong Kong Island, are very cheap, and you can see loads from them, we enjoyed the experience of going along Dried Seafood Street (yes, that's what it's called!), where every shop sells, well, dried seafood!; we also enjoyed the escalators that take you high up, they run downwards in the morning and upwards from about 2pm!
We rented an apartment in Kennedy (the end of the tram at one end of the island), which is traditionally one of the Chinese areas (rather than ex-pats) of Hong Kong. I would highly recommend that you go to the market there, we've never experienced anything like it before. One of our favourite places was also in Kennedy - the public swimming pool. It is free (or was when we were there), very clean (you have to walk through a shower and foot bath before you can enter the pool), but what a place. The swimming pool is outdoors and on a roof top. There are actually four pools, a learners one, a childrens one with slides etc, an olympic size pool, and the general one. It's such an experience to be in a pool on a roof top surrounded by all these high rise flats, we loved it.