You might look into Konall Culture Exchange, which is a very small (usually no more than 5 -10 students) school located in Shijiazhuang, about 2hrs south of Beijing by train. Their teaching schedule is very flexible, just tell them how much time you have and they will create a course for you from 1 week to 1 year. You spend 4 hours a day in class one on one with your teacher - no hiding in the back of the class! They'll find an apartment for you or you can stay with a host family and really go for total immersion. I was there for 5 weeks a couple summers ago and other than my fellow students I never saw another foreigner. One drawback is that the air pollution is as bad as Beijing, though it didn't bother me that much, perhaps because I lived in Los Angeles for 15 years.

Their website is http://www.gatewaychina.net/

If my wife would only let me I'd go back tomorrow . . .