yiyuanjing

As someone who grew up going to Sai Kung:

Sai Kung town has the better seafood - the restaurants along the waterfront are mostly touristy but genuinely good. The market is charming.

Sai Wan (the beach itself, reached via a 40-minute hike) is the real gem. Crystal clear water, white sand, basically no facilities. This is what you dream of when you think Hong Kong beaches.

Do both: eat seafood in town, then hike to Sai Wan. Best day out in Hong Kong.