China in 21 days
Days 1-13: visit Shanghai, Beijing, Xi’an, Jiuzhaigou, and Huanglong as described in China in 13 days.
Day 14: Jiuzhaigou/Huanglong-Hong Kong: there are no direct flights between Jiuzhai Huanglong Airport and Hong Kong, but only via Chengdu. You will have to spend at least 6.5 hours in transportation, including stop time in Chengdu.
Days 15-17: Hong Kong: see Three days in Hong Kong.
Days 18-20: Yangshuo and Li River: visit Yangshuo and Li River as described in Yangshuo and Li River travel guide.
Day 21: travel from Yangshuo to Hong Kong and back home.
More than 21 days
If you have more than 21 days in China, you can spend few extra days in Hong Kong, including day trip to Macau. There are also interesting activities, and roads less traveled in Yangshuo.