I just checked, and it looks like there's a bus from a nearby subway station out to the Toronto Zoo, and service runs every day until after six. So, since a membership for the family is $150 and a single trip without membership would be $60 before tax, that looks like a viable option. Space to run, animals, an overpriced gift shop to avoid, lots of opportunity to accidentally lose children. Perfect.
I think I might leave the girls with the monkeys to be socialized.