It will blow you away......

Just get on the metro and get off at Diamond Hill MTR Station on the Kwun Tong line. Outside you should see the signs to follow. The gardens are about 100meters away. There is no entrance fee. The visit will take you about 1.5 hours, including the Chi Lin Nunnery and the Nan Lian Gardens, technically another attraction, by a bridge.

If you are into ancient Chinese architecture and Buddhist art and peace, then the nunnery is a must, the building is beautiful. The nunnery is an actual Buddhist place of worship, so taking pictures or video are prohibited. Behind the nunnery is a small market that gives the impression that the revenues are for the nunnery. 

As for the gardens, if you like to take photographs make sure the memory of your camera has enough free space - you will need it. They are the finest Hong Kong has to offer and rival most landscaped gardens you can find in Japan, Korea or mainland China.

This visit is a great way to spend some time away from the bustle (although the roads go around and straight through it) of the city.

After the Nan Lian Garden and the Chi Lin Nunnery, back above the MTR station, there is a big shopping mall, called Plaza Hollywood, with a very good food court on the 3rd floor.