http://www.volunteercalgary.ab.ca/
They have a searchable database of volunteer opportunities. I would suggest looking for short term volunteer opportunities first so you can see if it's for you.
You might also want to look for an opportunity that allows you to expand your career. The search allows you to look at volunteer jobs for marketing, financial, board members, etc. Not all opportunities are frontline but are still necessary for the operation of these non-profits.