Take the cross-harbour ferry (day and night crossings)
Take the tram from Causeway bay to Central
Take the mid-levels escalator
Take the peak railway late in the afternoon so you can see HK harbour in daylight and nightime
Visit Stanley market, Aberdeen, Kowloon (Nathan Road - don't buy a watch!)
Get drunk in Wan Chai (not Lan Kwai Fong - too expensive and ex-pat) Wan Chai is way more fun.
Go to Central on a Sunday. Deeply bizarre sight.
If you've got any shopping budget, I'd recommend Pacific Custom Tailors in Pacific Place (3rd floor). Go see them as soon as you get to HK and get yourself some truly great clothes made. Yes, you can get cheaper elsewhere but you won't get better. I have suits I bought there ten years ago and they're still brilliant (if a little tight around the old tummy!) Their shirts are superb as well. Get one of the deals - a suit with a spare pair of trousers, blazer, two pairs of slacks and five shirts. Sorted for the next five years at least.
take a boat trip from Aberdeen to
Lamma Island. Quite fun, very laid-back, used to be full of travellers smoking dope but that was a while back. Fresh seafood there is very good.