Depending on your interests / fitness (and in no particular order):
Go hiking up 3000' mountains or any of the other thousands of kms of trails
Visit Tai O
Go looking at ancient rock carvings
Go spend a day at the beach - maybe windsurfing at Cheung Chau if the weather's right
Visit Kadoorie Farm
Go and explore the Lung Yeuk Tau Heritage Trail
Go rock climbing on Tung Lung Chau
Visit Po Toi Island and have the excellent chilli-pepper squid at Man Kee.
Go mountain biking on Lamma Island
Go birdwatching at the Mai Po Nature Reserve
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