One of coolest hiking trails in hong king is Tai Po kau Nature Reserve which is a short taxi ride Tai Po Market KCR Station

If want to see beautiful nature, get away from the crowds and experience the real beauty of hong kong.  Its highly recomended.  You can select from many nature trails which vary in difficulty from easy to moderate.  Its a Must Do if you live in Hong Kong and after you do it - It feels like you just came back back from a vacation.