I highly recommend BLCU (Beijing Language and Culture University) 北京语言大学. I have just returned from a semester there at the Intensive Institute 速成学院, 6 hours a day of excellent teaching. Absolutely worthwhile. They have study programs that last a month, 12 weeks, 20 weeks or a whole semester.

http://www.blcu.edu.cn/blcuweb/english/index-en.asp#