I second what StCrispin says - you should definatley go to Macau for the day from HK - high speed catamaran ferries take about 50 mins and depart 3 or 4 times per hour - cost about £12 return. Just buy a ticket at the terminal and remember your passport. Quite a walk into town from the ferry terminal at Macau. You'll find it's much more 'European' than HK - very relaxed - good shopping and food. I wish I'd stayed over the night - if you're in an expensice HK hotel this'll save you some money too.
While in HK ride the Star Ferries across the harbour as often as you can. Best time is sunset - watch the sun go down and the lights neon lights come on etc. Has to be THE best value tourist trip in the world at about 12 pence each way.
Don't forget to visit the beaches at the south of HK island - Stanley is the biggest town - apparently where many of the ex-pat Brits live (and where the tourists go for fish and chips a pint of Guiness or John Smiths ) - the market there is the best place to buy cheap clothes and souveniers. Not my cup of tea to be honest - if I wanted Brit food I'd stay at home, but certainley a pleasant break from the bustle of Kowloon. Doeble decker bus taken about 30 minutes.
Don't know when you plan to go, but I found November the best value - cheapest airfare and hotels and the weather is still pretty decent. Much earlier and it'll cost more and might be wetter; later and the weather will be cooler. There seem to be lots of hotel deals (no single suppliment etc.) about this time.
Whatever you do, enjoy the trip.