Maybe coursera or some other moocs. There is a mooc called 学堂在线. It is a free platform builded by Tsinghua University. It is a very good mooc. The teachers on it are all great professors of Tsinghua University. I guess there may be some free course which you want on it.

Actually it's hard to find a platform you required above,you can try to make friends with chinese and have a chat in your free time,Isure you'll learn a lot.

there's a website names 网易公开课，http://open.163.com/cuvocw/  this is the link of the website. You can find the courses which are given by the Chinese teachers from different universities on various topics, mostly in Chinese(Mandarin).