10 day Honeymoon - Need help getting started with planning

My finance and I are hoping to travel to China for our honeymoon in mid-May. We will have about 10 days to explore the country (starting May 7th). We definitely want to spend a few days in Beijing and Xi'an. We are not much interested in Shanghai; just seems like a business city. What we can't quite decide on is what other location to do. Want to get out in nature and see some of the iconic mountains. A few places that look cool are Yangshuo, Haungshan, and Zhangjiajie. Which of these would be most feasible for to fit our itinerary? Is Zhangjiajie way too crowded? What would be best way to return to the United States after this? Going back to Beijing doesn't seems like a good use of time, but I don't know of other options. Would you recommend utilizing a travel agency to help coordinate all this?