I would not advise the bus. I assume that your bags are checked all the way (otherwise check them immediately to your final destination). You probably wish to leave your hand luggage at the airport. I let som experts tell you where it can be stored.
Then take the train to Hong Kong station (buy a same-day return ticket). From there I suggest that you walk to the Star Ferry pier. If you have never been to HK, the ride on the harbour between Hong Kong Island (Central) and Kowloon, back and forth is the most wonderful view of HK, hassle-free and at very little cost. Actually you could even try to stay onboard and come back with the same ship. It is illegal, but I've done it many times. Walking around Hong Kong at that time of the year is fun. From HK station, you should try to walk uphill (south) to get into the older part of town, holywood street, open market streets, etc... Spending 3 hours is great. And HK is evry safe and efficient. If you get lost, its easy to get a taxi.