my wife and i have 20hr layover Baiyun Airport, kindly suggest how can we fully use of this time.

If you arrive in the daytime, you are suggested to make use of this time to explore the city a bit. You can take subway to downtown area and the ride takes around 1.5-2 hours for one way. You can visit the Shamian Island, Beijing Road Pedestrian Street, Shangxiajiu Pedestrian Street or Teemall, Zhujiang New Town and surrounding area. They are all good places for shopping or dining and can be easily accessible by subway.