All great suggestions above. Here are some other notes -->
Downtown Activities
1. Jays game. I would think kids would find the baseball game very cool, though may get bored near the end (which is normal). But it'd be a great memory. There aren't many bad seats, even sitting up in the 500s is quite the view (if you are ok with heights). If you want the cheap seats (ie 500s), and the dome is open, I'd sit around the 516s if you want sun and/or 533s if you want the city view.
- It looks like the Jays are playing the Rangers (a rival) at home on the 26th. On the day of, go here to know if the dome will be open that night (
http://www.isthedomeopen.com/)
2. CN tower is also recommended
3. The aquarium is world class. You have to go there as well. I would recommend trying to avoid the crowds when you go, so go off hours. Either right when they open (9am), or at night. You could go after dinner and walk through. It closes at 11 (except on May 24th, closes early)
4. Hockey Hall of Fame. Every member of CP owes it to themselves to go. You can prioritise the Cup room. Average visit is about 2.5 hours, so you can shave a bit of that when in a rush.
5. The St. Lawrence Market is only about 2 blocks east of the HHOF. Good for a quick walk through and a bite.
6. Toronto Island ferry would also be a good trip. Great view of Toronto
(you could probably do 4,5,6 easy in a day. Get to HHOF at opening, go to St. Lawrence for lunch, head to the island for the afternoon. If it is raining, do CN tower or aquarium - however, I would recommend you do aquarium at opening or much later)
Non-downtown
7. Casa Loma. Not far from downtown. An old castle in the city that has been restored. Cool to walk around in. Not exciting, but kinda neat.
8. ROM. Good for a rainy day. It is a museum, but fairly interactive and huge.
9. AGO. If your kids like art, it is also world class.
10. Canada's wonderland will take a full day, due to location and stuff to do. I'd recommend you budget for the Fast-Pass. Will save you hours of waiting in line. Your kids will love it. There are some SERIOUS coasters there.
9. The Toronto Zoo. Also out of town, so budget a solid 3/4 day.