One of HK's main attractions is its skyline. And the price is free!
Get down to the TST Star ferry terminal or anywhere along the south bank of TST for an amazing view. Best seen at night.
At 20:00 each evening there is a "light show" where somehow they've set up the buildings of the HK skyline to have a light show in time with music. There are lasers too. And it's free. Definitely get down there.