Hong Kong doesn't have great beaches, but they're ok. If you're going further afield, the beaches at Sai Kung can be nice and isolated - otherwise, I think Repulse Bay or Shek O (be sure to eat at the Thai Restaurant that no one knows the name of) should be fine for your needs - and those are only a 30 minute bus ride away on the south side of the island - as opposed to a 30 minute MTR ride + a 25 minute taxi ride for Sai Kung.