i sponsored my parents for permanent residence to canada. I was already approved as sponsor and their application is currently being processed in our home country. They are now here in Canada for a visit, but would like to do some part time work (they just want to do something to keep them busy). I tried looking for volunteer work, but the volunteer work I saw still needs work permit.

Would it be possible for my parents to apply for open work permit?

If either obtains a specific job offer, then yes it is possible for your parents to apply for a work permit. If either is granted a work permit of longer than 6 months in a high-skill occupation, then it may be possible for the other to seek an open spousal work permit.