If you travel to Beijing, you must see the Forbidden City, but before travelling to Beijing, read Twilight in the Forbidden City by Reginald Fleming Johnston - recently republished and available on Amazon. It will give you a sense of history and the politics of the Forbidden City.

The crowds can be somewhat large, but its great to do this in the morning then exit at the north gate and then head west to Beihai Park.

The audio guide (a recording) that you can rent is very helpful!