Three weeks? If so, I think you will have enough time to visit the essence of China, such as Shanghai, Beijing, Xian and Guilin. Or, would you just like to visit Shanghai and its nearby places, such as Suzhou, Hangzhou and Zhouzhuang. Please give me a brief idea firstly~

3 weeks is enough for also other places around SH only need ur well organized. I hv good suggestion to stay not inside Shanghai where now is crowded and expensive caused by Expo but in smaller city nearby.In there also very convinent to tk bullet train about half an hour to Shanghai and city like Hangzhou,even less time than taxi from airport to downtown of SH.