CAN A VISITOR IN CANADA WORK AS A VOLUNTEER ?

The answer is not always clear. In general, the rule is that if you volunteer at a job that a Canadian person would normally be paid ( regardless of whether you’re actually paid or not) is considered to be work (and unauthorized for a visitor).
As an example, handing out pamphlets as a church should be fine but consulting for free at a company or the zoo would not be permitted.