I recently went to the Brockville tunnel that's in the book of things to see in Ontario. It has just been renovated and opened to the public as a free attraction. It's actually pretty neat. Probably a long way to go for some from the GTA but if you're heading to see the ruined abbey north of Cornwall that's in that book too it should be on your list. I'll post some pics up soon when I work out how to do it.