Go to Shenzhen. It&;s near and convenient.
Watch the sunrise and play barbecue by the sea
Tung Chung, Sai Chung and Yangmeikeng Valley, choose one of three. I recommend Tung Chung for sunrise, Xiyong for swimming in the sea, and Yangmeikeng Valley for scenery.
There are many water sports, so you can choose freely.
Xiyong is said to be one of the eight most beautiful beaches.