Because of the long weekend there should be a few groups about, if your on Facebook join the MTB.HK group, there's usually rides listed an visitors are always welcome, relativity easy to rent a decent bike for the weekend if required.
The guys at HKMBA (Brandon/Kenneth etc) are good guys to know and know all of the trails, good people to hook up with. If your up for some riding over the two weekend your welcome to PM me as well, email in my profile.
Even though its starting to chill a bit here I recommend a minimum 2-3 liter bladder, it does get hot when you start riding and can be a pain if your not used to the humidity.
Regarding the tourist side, most has been listed above, the minimum you should do is the Star Ferry across Victoria Harbor and a trip to The Peak.
Public transport is cheap and the MTR is one of the best transport system in the world, plus you can take a bike on it (just remove the front wheel).
Recommend you get an Octopus card at the MTR desk at the airport arrival hall, this is the same as an Oyster card but also enables you to purchase good in some shops so useful to have one, you can hand it back and get your deposit returned when you leave (deposit is 50 HKD).
Enjoy your stay, Hong Kong is one of the most amazing cities in the world to live and has a surprising amount of country parks to explore as well.