The CN Tower is the rightful pride of Canada, standing 298 meters above the ground and overlooking the world class city of Toronto. It is right downtown, so if you have a couple of free hours it is worth the trip. Be prepared for a waiting line, and a quick security check. The area has lots of cafes, restaurants, boutiques and galleries, so there is plenty of entertainment you can package around the verticality of the Tower.
Go during the day to see the city spreading out in all directions, and Lake Ontario lapping to the south. Toronto is not a tall city by any means, but it has great charm, character and cultural events.
Definitely worth it.