We (me, my partner, & 10-year-old daughter) are headed to Toronto next week for 6 or 7 days.
None of us have been before and would love any advice/recommendations. Things we know we’re doing: a day trip to Niagara Falls and a trip out to Toronto Islands to bike/canoe around.
We’re from DC so are very used to using mass transit; I gather rush hour in Toronto is just as bad as it is here. Our hotel is near the Harbourfront if that helps w/recommendations.
Day trip to Niagara Falls: if it’s on a weekend, you can take the train. This is a new service that the regional train system, GO Transit, is trying out on the weekends. There’s also regular intercity VIA trains there, which may be marginally more comfortable.
The Islands: a wonderful picnic destination, with an amusement park and everything. Unfortunately, there’s a city worker strike on, which means that the ferries there aren’t running and the park is closed. 
(Special arrangements have been made, I believe and hope, for the people who actually live on the Islands.)
What things are you interested in? Museums? Art? The Science Centre? The Hockey Hall of Fame? Shopping? Meeting members of the SDMB? 
Oooh, thanks for the info about the city worker strike.
The science centre sounds interesting, but does it have stuff that’s significantly different from the Baltimore science center? I do see that they have a lizard & snake exhibit right now, which is right up my alley.
I am a museum person but SO & kiddo are not so much, so if there is something really out there/unusual I could see about coaxing them into it.
Hockey and shopping are no and no…
How about a shoe museum? 
Welcome! You’ve picked a great time of year to visit… there’s tons of stuff going on around the city this month.
I also second the motion for the Bata Shoe Museum. It’s really quite entertaining as far as museums go, though it’s rather small… best if you combine that with a visit to the Royal Ontario Museum, which is a 5 min walk away. They’ve got a couple of great hands-on areas for the kids to play in.
Assuming you’re here on a weekend, I’d suggest checking out the goings-on at Harbourfront Centre. They’ve got fantastic music and performing arts festivals on the weekends all summer long. The Saturday night headliners are almost always worth the while (schedules and lineups here). It’s all free, and they’ve usually got a few things on the agenda directly specifically at the kidlets.
The Toronto Zoo is pretty cool, but unfortunately it’s a bit of a haul on public transit since it’s on the outskirts of the city. Lots to see and do… you could spend a whole day there and still not have seen everything. Best to do on a not-too-hot day since the animals will be more active instead of passed out in the nearest patch of shade.
You may also want to check out Ontario Place, which is easy to reach from your hotel (there’s a streetcar that runs directly from Union Station). There’s a huge water park that’s great on hot days, and several other rides that would be suitable for a 10 year old (I heartily recommend the Wilderness Adventure Ride, which has a high cheese factor but is incredibly fun)
If you’re going out towards Niagara anyways, you’ll pass by Hamilton, which I don’t really recommend for a visit with the exception of the Canadian Warplane Heritage Museum. It’s a very fun museum with planes you can climb around in.
Thirded.
When we go to Toronto, it’s to visit relatives, so I’ve missed a lot of the main attractions. We spend most of our time talking and eating fantastic food from the conglomeration of cultures. We did make it to the Bata, though, and it was worth it. Way fun museum. My niece’s favorite bit was the astronaut boot.
If you find yourself at Eaton Centre, go out into Trinity Square and see if the Church of the Holy Trinity is open. Interesting building. Half of it burned down some time ago and was rebuilt - the stained glass on that half is different. The acoustics in the church are semi-famous, and musical groups (the Cowboy Junkies, for one) sometimes rent the church for recording sessions. Not a huge destination, but if you’re passing, stick your head in. If I recall correctly, there’s a lovely labyrinth in Trinity Square as well, if you’re interested in that sort of thing.
Have fun!
What, no suggestions for the CN Tower?
It still amazes me to go up and see the view.
And don’t forget the elevator with the glass floor! 
Best way to get to the zoo on public transit? Take the GO Train to Rouge Hill station and take the #85A bus straight to the zoo. Much better than the hour-long slog along Sheppard from the subway.
Note: the streetcar runs to the north side of Exhibition Place, at Exhibition GO station. You have to walk south through Exhibition Place to get to Ontario Place. It’s about 15 minutes. You could get off the streetcar on Fleet Street and cut through Coronation park and get to Ontario Place that way, but it wouldn’t be any less walking.