Well... If the weather is sunny, and there's no air pollution,
you can stay in HK Island and Kowloon for your whole trip.... (HK
Island and Kowloon already have many many attractions and places to
shop and eat.) If the pollution is bad, you can go to the far-away
places which everyone mentioned... (Lantau, Disneyland, Shatin,
Shenzhen, Macau, etc)
For the far-away places, it's advisable to choose only one or
two... because transportation is expensive in HK and travelling
time is long. (If you wanna go Macau or Shenzhen, you still need to
consider the time taken to clear immigration... Waiting time of
more than one hour at checkpoints are not uncommon.)
If time is limited, just focus on HK Island + Kowloon (+ either
Lantau Buddha and/or Disneyland).... Next time, you can always take
Tiger Airways directly to Macau or Shenzhen. 