Comfortable shoes - Especially if you have big feet! You’ll be doing a lot of walking here, and you'll struggle to find larger shoe sizes in China.
A transformer - Most wall sockets in China will accept European, North American and Australian plugs, but Americans beware! Voltage here is 220V, so you'll need a transformer if you want to avoid frying appliances that run on USA voltage.
Toiletries - Bring enough to last your trip, especially if you're precious about the kind you use. You may have a hard time finding familiar brands here.
A scan of your passport - Either stored on the Cloud or backed up on your phone or USB key. The same goes for other important details like credit card numbers.
Space - Make sure you leave enough room in your luggage for all those gifts, souvenirs and trinkets!