21 hrs transit in Beijing

Chan, you did not mention your citizenship. If your citizenship is a Malaysian one then you should know that you can enter China absolutely visa free for up to 30 days. In other words, you are free to exit airport as you wish and to enjoy your time in Beijing exactly in the way you like. You should just have in mind to take an arrival card before your landing in Beijing that will be dispensed on board the plane. You will fill out the arrival card and tick the box "transit". When you land, you will head directly to the immigration with your passport and the filled out arrival card. Enjoy in your stay in Beijing.