Generally speaking, Mid-May might be one of the best time in Xi'an. It'll be not too hot nor too cold. It will take you 2-3 days to visit the most general scenic spot in Xi'an ,just like the Bell Tower,the Drum Tower, the City Wall, the Qin Terra-Cotta Warriors and Horses Figurines ， the Shaanxi historical museum, the Stone forest museum and so on. Beijing, the capital of China, is also a city with a long history. It's quite convenient to go to Beijing from Xi'an. It'll take about 12 hours by train and 1.5 hours by air.

Hi,Ms.Cheng,
May is the best season to visit Xi'an. the temperature normally about 24C or even warmer, don't forget your sun glasses. you can stay in Xi'an about 3 or 4 days, and you can visit Beijing also. Shanghai is commercial center of China, very western and mordern city. If you only like the history and beatiful scenery, you can visit Lijiang or Yangshuo instead.