I climbed Lion Rock during my first hiking in HK. It lies on the MacLehose trail and stands up 495m above the sea level. From the peak you get two beautiful views: one on Sha Tin, one on Kowloon Bay.
Lion Rock Park is well known for its macaques and there are signs warning people not to get too close to the animals at the park entry. Funny thing was, that I have seen just one of them while I was in the park, and was approached by a group of 20 when I thought I was already back to the city.
You can read more about that hiking here.