My wife, three teenage sons and I will be going in a few weeks. Any suggestions on things to do and see?
The Toronto Zoo and the Royal Ontario Museum are excellent. Casa Loma is great if you like historical homes. If you’re looking for something that isn’t just a tourist destination, you can take a walk through Chinatown.
I went there a few years back and I really enjoyed the more ‘educational’ spots. There’s an amazing aquarium right next to the CN Tower, which in itself is impressive. And the Royal Ontario Museum was just fantastic, especially if you like dinosaur bones. Take some chances with small, local eateries too. Oh and if you go to the CN Tower and see a guy dressed like a Tibetan monk in the plaza, avoid him like the plague lol he’s just after your money.Find that out the hard way.
I didn’t care much for the ROM. I don’t care for looking at old pots and pans from antiquity. But the ROM is huge and there may be portions your family would be interested in. Check it out to see if it’ll be worthwhile.
Go up the CN Tower. Pick a day with good visibility.
See a Blue Jays game at the Rogers Centre and cross that stadium off your list of the 30 MLB ballparks. What list, you ask? Start one! Get the MLB Ballpark Passport and get your first stamp for the Rogers Centre. {https://mlbballparkpassport.com/} It is a lifelong journey. I visited my first stadium when I was 8 or 9, and I am now 55 and just got stadiums 28 and 29 this week. Only one to go. The Blue Jays are playing well and their fans are excited.
If you haven’t tried curling, find a club that’ll give an intro lesson. It was a very fun family thing for us when we did it just once. It might give you a break from the heat if it’s a hot day.
Consider a day trip to Niagara Falls, a must-see in my book.
Have fun!
Consider taking your teenagers on a walk. CN Tower Edgewalk.
British/Canadian/American history at Fort York where you did bad things to us, and any number of small museums that are windows into Toronto’s past.
For live shows (symphony, jazz, opera, ballet, theatre large and small), there is a lot going on in Toronto.
Toronto has quite a few recreational trails (and several places that rent bikes), and quite a few beaches, including half a dozen on the Toronto Islands, which are accessed by ferry.
This is not strictly a “Toronto” suggestion but if you’re coming by car or will be renting one and feel like getting out of the city for some fresh air and scenic vistas, consider a trip out to wine country to explore Ontario’s burgeoning wine industry. Tour the wineries, enjoy tastings, and have lunch at a winery.
The closest area would be the Niagara Escarpment and Twenty Valley, about 80 km from Toronto. You can have anything from a fancy multi-course gourmet lunch with expertly matched wines at Vineland Estates to an informal charcuterie on the outdoor patio of Featherstone’s old farmhouse or casual Italian food at the Ridgepoint winery. The main fun is seeing the wineries and the beautiful scenery, tasting their products, and picking stuff up to take home. You’ll want to get the Wine Country Ontario Travel Guide, free from any tourism office or any of the wineries.
Add the Ontario Science Center to the list.
I’ll second the zoo. You do want to pick and choose in the ROM; maybe look at its map and decide what parts you’re interested in. And I really liked Fort York too.
I’ll also second the TTC. You do not want to drive and park around central Toronto. Leave your car at the hotel, except maybe for the zoo, because it’s a long haul on the bus.
And for teenage boys many of the interior scenes for Xavier’s mansion from the X-Men movies were filmed there. It also makes an appearance in Scott Pilgrim.
For professional interest, see Osgoode Hall (the court house downtown, not the law school way north). The Court of Appeal for Ontario sits there. It’s an mid 19th century court house, surrounded by a green lawn, and cast iron fence. When it was built, the area was rural and there were cows wandering around, so the gates are fixed to keep the cows out. No cows now, of course - it’s in the down town area.
It’s also the seat of the Law Society of Upper Canada - note that Upper Canada has not existed since 1841, but that’s no reason to change the name of the Law Society.
You could take in a CFL game, if there are any football afficiandos amongst you. Sadly, the Roughriders won’t be playing in Toronto until later in the season, so you won’t see the CFL at its finest, but I understand there is a home team there that you might want to watch. 
…built on the site of an insane asylum. 
Pic of the LSUC’s Great Library.
Across the street (Avenue Rd.) is Chief Justice Campbell’s House (one of those mini-museums with really well informed staff). In 1972, the Advocates’ Society hired my dad to move it to where it is now when the Hallmark Card company wanted to tear it down. Yay dad! Yay Advocates’ Society!
If you’re driving up via Niagara Falls, you could do a side trip to Niagara-on-the-Lake. There’s wineries in the area, but also the Shaw Festival. Niagara-on-the-Lake was the first capital of Upper Canada in the loyalist period (1790) but I don’t think there are any historical buildings there. The plays are good!
And the walls of the Government and Opposition caucus rooms are both covered with scrawls: “all talk and no votes makes Jack a bad boy”
And of course you could all get your picture taken underneath the road sign for Avenue Road. (Torontonians lack a bit of imagination sometimes… 
)
Watch out for the fake monks in the square in front of Union Station. Not monks at all, but persistent, aggressive beggars, and not the down-on-their-luck types.
Your Dad was in the house-moving business, Muffin? Good on him for saving a historic property!
Why did Hallmark want to tear it down?