I was in Beijing for 3 days and have to say that the two best experience for me was the Great Wall hiking and Beihai Park.

The Forbidden City/Palace would be great if not for the thousands of people swarming there where you can't find any personal space...

If you can do you yourself a favor and spend at least a full day at the wall hiking which will make you jet lag go away faster and you will not feel as exhausted when you look at all the beauty around you.