Hong Kong is part of the karst limestone mountain range that runs through southern China, through Vietnam (Ha Long Bay), Thailand, and the Philippines. For such a small geographical area it packs quite a bit to see between beaches, mountain treks, and unusual natural formations. There's also usually a good restaurant at the end of every hike.
I was in Hong Kong from October 2014 - February 2015...so unfortunately I did not have the opportunity to properly enjoy any beaches.
There's always a hill somewhere you can climb. Many of Hong Kong's hikes are also easily accessible via public transit. The most popular and easiest hike is Dragon's Back.