The main activity of Hong Kong is concentrated on the actual Hong Kong Island. Though it is so busy with people and traffic, that apart from some decent hiking, there is very little space for natural environment and for the day out on a bike.
There are New Territories - slightly scary Terra Incognita lying beyond the Kowloon. Not every ex-pat based in Hong Kong Island is courageous enough to visit New Territories. But is you love cycling I would strongly recommend to take MTR (or taxi) to Tai Wai (or Sha Tin). You will find plenty of bike rental shops with huge choice. There are about 60 miles of cycling paths lying from Tai Wai up to Plover Cove and back on the both sides of the picturesque canal. Paths are safe and pleasant, there are plenty of places for the pit stops, with the Science Park on its way with its restaurants and coffee places. Some shops do have a handy option of two locations - you hire a bike in Sha Tin (or Tai Wai) and you can hand it back in Plover Cove (or Tai Po).
I would suggest chose mornings for your time out. It is getting extremely busy towards the afternoon. Pretty empty during the week though, if you can do that.
In terms of timing it will take about 30-40 min on MTR from Central to Sha Tin or Tai Wai and about 1,5 hours of cycling from Sha Tin (or Tai Wai) to Plover Cove.
In terms of money average bike rental price is HKD50 for the day
Enjoy your day out!
