I will be in beijing this april for 6 days, advice some other town where to stay other than BJ.

Oh, you are suggested to go to Tianjin. This ciy also has many must-sees. The distance between BJ and Tianjin is just about 85 miles.
You can go to Huangyaguan Great Wall, Dagu Fort, Ancient Cultural Street and Dabei Buddhist Temple. In addition, Mt. Panshan Scenic Area, Baxian Mountain and Seashore Holiday Resort are also good choice for you.

Xian has the Terracotta Army. I would suggest a train there. On the way is a mountainside walk to a temple! It is an easy path. If you like outdoors, it is near Luoyang i think. Shaoling Temple is the name of it.

I recommend Shanghai. Today Shanghai is a modern, relatively safe, and fascinating city with an energy and culture all its own. During this time Shanghai established itself as a major hub for business and international tourism. This mixed cultural history can still be seen in the architecture of the city today. April is the best time to tour Shanghai. The temperature is comfortable at around 20°C and there is little to no rain.