Forbidden City the other "must do" in Beijing is the Great Wall of China, which is a few miles north of the city.
Even more than the Forbidden City it was overwhelming in its scale. You can see its line dotted with forts string its way up and down hills as far as the eye can see, and yet you can only see a fraction of it at any one time.
Apparently you can't actually see it from space as its not wide enough, but boy is it long. According to Wikipedia, with all its branches and counting those bits that have weathered away, there could have been as much as 21,196 km of it.
I did a hiking tour from Simatai to Jinshanling which was about 7 km with lots of up and down so it was a good thing I had three bottles of water with me.
You will be defeated, eventually, simply by the scale of the Great Wall.