Sadly, volunteering isn’t a very “Chinese” thing. I used to be a very active Rotary Club member, but felt a little culture-shock that there is no volunteering here. Here are some of the activities you can join if you are interested still.
1) Satellite Rotary Club for Beijing (Please PM)
2) Toastmasters （If you look on the Toastmasters, site you will see there are several clubs here, however quite a number of them are company-exclusive)
3) Church: I know there are several church in Chengdu. There is Christian service near Tongzilin every Sunday. There are also Morman services here. (Please PM)
4) As Chris mentioned, there are also a number of English Corners. There’s a group that meets on Friday, and another Sat/Sun (I forget), but I’ve never been to them.
5) Couchsurfing