Just seen this so hope its in time to be useful. I assume you'll visit the wall and my suggestion is Mutianyu, and to visit in the afternoon rather than join the throng of coach tours in the morning. This was our guides' suggestion upon learning that I was a photographer. We visited after a windy storm, and had the section virtually to ourselves in great light.
In Beijing I enjoyed the Temple of Heaven- especially the ceiling of the main tower, and in particular the Lama Temple which had far fewer people than the Forbidden City.
Finally the Terracotta army. In the pits you'll need 200mm to get a head to feet shot of the nearest soldiers. Obviously you'd get away with a shorter lens for the figures in glass cases if you can use the light. More than half my photographs were made at or close to 200mm and at high ISO to give some semblance of depth of field.