coco,
For a day time trip, i would recommend the
Ngong Ping 360 This a very scenic cable car ride to the giant budda on top of the mountain. It's close to the airport so not a long way off.
Trains to the city are every 20 mins and take 20 mins to get to HK island or 15 mins to Kowloon. There are plenty of things there, bascially a walk along both sides of the harbor is a good experience.
HTH.