Hong Kong is a city more than anything else: big buildings, traffic, lots of people. But there are places that allow locals to commune with nature and get a bit of exercise.
This is one of my favorite trails in Hong Kong:
Dragon's Back -- Thumbs up for this very easy and quick climb. The view of the hills remind one of the spine of a Chinese dragon, hence the name. Since it only takes around 2-3 hours to finish it, there's no problem about packing the right gear. Water, some snacks, a good pair of shoes and a camera (of course).