Download VPN and Maps.me
if you have no roaming data card of your country with your cell phone, download the VPN and maps.me in your country in advance. otherwise you will not be able to use the google map, facebook, twitter, instagram, gmail, etc.
if you are layover in PEK airport, you should print your next flight tickets, hotel or tour booking, if you store these info in your gmail, you may not be able to open them.
after downloading the maps.me, you should also download the maps of beijing or other cities in advance.