This is a bit different than what you were thinking but you can't work out something within the school it might be an idea.
Here's a group that organizes activities on weekends, one 6 week session in the spring and another 6 weeks in the fall. They would have to have some sort of insurance to rent space as MON mentioned. If you click on the brochure at the bottom of the page you can see the types of activities they run.
http://www.abcontario.ca/ottawa/44/I've heard good things about the program and chances for kids to meet up with others that have similar interests. I've also heard parents praise the impromptu parental discussions that go on in the hall while the kids are in the activities. We tried to sign up last year but we were ~200th in line and the one DS wanted was full before we got to the front. Since then DS hasn't wanted to do it so I don't have any personal experience. They are very popular and I believe the main thing holding them back from doing more is lack of people to design and run the classes.
