Explore the whole of East Kowloon and Hong Kong Island, it's definitely no regrets for sure. If you find the weather in the afternoon too blazing hot, take a walk in the evening or the night where it's absolutely lovely. One must-visit location that I went was the Tai Tau Chau, just off Shek-O beach (Visit it in during the weekdays to avoid the crowd, there was barely a pack during the weekday). It's a simple yet somehow most frightening bus ride away from Shau Kei Wan where you can just board bus 9 and it will take you right to the beach. Go exploring just off the east side of the beach where the local houses are (the houses are probably owned by millionaires I believe), right at the most extreme east is an island called Tai Tau Chau where it is about a 10/20 minutes hike up to the top where the view is absolutely gorgeous, and not to mention that cold sea breeze... Best surprise location.