What to bring and what not to bring to the Forbidden City
What to bring:
Passport: it's needed for foreigners to buy tickets
Cash: for paying for your tickets.
Backpack: to carry all your loose items
Water: Forbidden City is huge. You need 4-7 hours inside to see it well. I've seen little shops inside where you can buy snacks from but they are few and far in between. Better bring your own water.
Snacks: you will likely get hungry. Buy some snacks
Sunscreen: if you visit in the summer, be sure to put on sunscreen as the palace is mostly outdoors
What not to bring:
Lighter: it will get confiscated at the entry security
Knives: it will get confiscated at the entry security
Explosives: you will probably get arrested
Anything other than water that you would not bring onto an airplane