Rent a bike near Sha Tin or Tai Po and cycle the Tolo Harbour Cycling Track to Tai Mei Tuk. It's completely flat, follows the water, and gives you countryside views 30 minutes from downtown Hong Kong.

The full route is 20km but you can turn around anywhere. Bike rental is HK$50-80 per day and the path is well-marked. End at Tai Mei Tuk for seafood restaurants by the water. Take MTR East Rail Line to Sha Tin or get off at Tai Po Market (Exit B) for easier access to the cycling track sections.