I came across this website while doing some searches on volunteering opportunities in Hong Kong. I think it’s quite up-to-date coz I found one of the events I wanted to join missing after one day!!! Check it out if you have time!
This one seems to be a big one in Hong Kong, coz they got a nice website. ;)
There is another one called Hands On Hong Kong. This organization gears toward bilingual people. Their English calendar is more updated than their Chinese one.