Don't do research because I already did it for you. If you're in Toronto for a day, try the following*:
Start in Kensington Market, a diverse neighborhood with vintage shops and boutiques. Have breakfast at Fika Cafe.
CN Tower: One of the largest structures in the world. It's taller than the Eiffel tower. Go early and make a reservation. Optional: Ripley's Aquarium - Toronto's city aquarium
Lunch: St. Lawrence Market - Toronto's main public market. Many eating options.