Longqing Gorge is a better known recreation area, it’s perfect for keeping cool in the summer. Chengde Summer Palace is an old favorite haunt of Emperor’s. It has stunning architecture.

Datong is great!

You could check out Cunadixia Village just outside Beijing. It's an ancient little town with well-preserved Maoost propaganda slogans still visible on the building's walls. It's quite tricky to get to though. There's some good advice in this article if you plan to go:

http://www.echinacities.com/beijing/city-life/your-ideal-escape-from-beijing-cuandixia.html