The ROM is a great, family-friendly option to take in art, culture and history. It features a terrific selection of fossils, revolving artwork from across the globe and interactive exhibits. We've successfully baby-sat Ben's nephews here before - so it's been kid-tested.
You could spend a whole day here. There are the incredible restaurants (we love Pure Spirits for the oysters), fabulous shopping (HWBP used to be stocked in a few stores) and a terrific performing arts centre. That doesn't even include all the historic Distillery stuff you could do - which embarrassingly we haven't gotten round to exploring yet. (Too busy trying out those restaurants.)
We've always loved this place but it wouldn't be on the list if Tom and Mei-Yu (brother and mother of the bride) hadn't also given it the designation of "best restaurant they ate at" in Toronto. Come for the French cuisine, stay for the ambience. Also a terrific spot for star spotting during the Toronto International Film Festival.
Kensington Market has something for everyone - food and drink from across the globe, vintage stores full of odd paraphernalia, jazz bars, dive bars, coffee shops that turn into cocktails bars. If you're in town on a Sunday, the streets are blocked off from traffic and turn into a pedestrian-only zone.