You can spend at least a half of a day up at the Peak as there is plenty of things to do and see for the whole family. However, it is also a very popular tourist destination so expect large crowds. It is best to go early in the morning. The views from the Terrace from the Peak Tower of the Island and Kowloon skylines are worth the trip alone. However, if you are feeling energetic, there is the "peak circuit trail" which is a 2.8 kms long paved path which is relatively flat. However, the views from this path of Kowloon and HK are awesome and, in some ways, better than those from the Peak Tower.