Here's a hint ...
Clearly, autumn is the best time to visit ... air is clean(er), sky is visible (and blue), and no sand storms from the Gobi desert. I like !
This section I visited today is the Mutianyu section, less crowded with visitors, and in a better state of conservation compared to the ones you'll find in Badaling.
A few more photos in my photoblog at http://mozphotos.blogspot.com.