I'd recommend going to the Fragrant Hills Park and walking to the top of the mountain (or you can take the cable car if you wish). As for the Great Wall, I found that the Jiankou section is a lovely quiet place to go as opposed to Badaling, for example, which is probably best avoided.
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.
Am back. Thanks to all for the tips.
I too enjoyed the temple of Heaven and in fact the park that is part of the area. Interesting to see Chinese couples practicing ball room dancing.
Basically , used my 70-200 with a 2x extender for at least 60% of the trip including the Warriors at Xi' an. I ignored the figures in the glass cases as I didn't have a polarizer with me and the crowds made it a lost cause. There is always a good use for postcards someplace along the way.