Technically this is not a secret, but practically it does not seem to be known much. The Wan Chai Ferry Pier has three levels – the first floor is the entry point to the Star Ferry to Tsim Sha Tsui and the Star Ferry Harbour Tour. On the second floor is an event location and the top floor is a public rooftop.
Just take the stairs or the elevator up to the top floor, and then you can enjoy a great view over the harbour. All for free.
There is a covered part, with a few flower beds – well, technically not flower beds, they are filled with cacti (or cactuses) and various dry plants that can resist the heat and live with minimum amount of water.
And a part that’s fully out in the sun or under the night sky, depending on which time of the day you go and visit. Either way, there is plenty of seating and you can just relax and enjoy the skyline.
We’ve been a number of times, and usually we bring some drinks (or ice cream from a Seven Eleven/K Circle at the Sun Hung Kai Centre) with us.
The kids can just run around (there are usually not many people) and is a great place from which to see the Laser/Lightshow if you want to come that late.
It is probably also a good vantage point for the fireworks, but so far we’ve never been.
bluebalu 
Thanks for the post. I am one of these people who didn’t previously realize that this rooftop garden was open to the public! :)
Hahaha, yes I can imagine that not many people know about this – and I was contemplating on whether or not to reveal it :)