The butterfly conservatory in Niagara is lovely, as is Bird Kingdom (Canadian side). There is also a military museum if your husband is a buff, but likely not so thrilling for the kids. If you have the time, Niagara on the Lake is stunning just to walk through for the day, and the area is surrounded by wineries and orchards.
In Toronto, the current hot ticket is Ripleys Aquarium. Only advice on that is get your ticket ahead of time (to avoid waiting in line). It's at the base of the CN tower so you can catch two attractions at once. Fort York is pretty close.
If the kids need to just burn some energy, Centre Island is just a ferry ride away, and usually there is a bunch of cool free stuff going on at Harbourfront.
The ROM is great, as is the Science Centre.
Casa Loma is gorgeous, but probably not super thrilling for kids.
The zoo is also amazing, but sadly it's a full day excursion on its own. I think the pandas are there this year, though.