The Forbidden City is incredible! It's so vast, intricate, a true architectural achievement full of art and history. My recommendations: buy tickets in advance (as early as possible), go early in the morning as you will easily spend a few hours there without even realising, get an audio guide (if your ticket does not have it included), walk around the side paths and palaces (west and east), not only the main ones - it's a lot less crowded and there are many amazing things to see. Once you go out, make sure to visit Jingshan park (it's right across the street) - there are great views of the entire palace and you can get an actual sense of how huge it is.