I'm assuming you'll have a car...
Well, it's easy to give a few sights - the CN Tower, Ripley's Aquarium, Toronto Zoo, Royal Ontario Museum. It bored my husband to tears (payback for the Warplane museum) but we did the BATA Shoe Museum. St Lawrence Market was nice. I wasn't overly impressed with Kensington Market - a little too...gritty...for me and we didn't stay long. Didn't get to the Distillery District, but I guess it's nice (it's near St Law Mkt).
If you had time, Niagara on the Lake is a great place to stop for a few hours to wander on the way to the Falls - some great shops. Niagara Falls itself is very touristy - kinda like Times Sq north...but maybe one less touristy thing would be to visit the Botanical Gardens and the Butterfly House. Skylon Tower gives some nice views. Of course, Maid of the Mist (which we didn't do either trip) and I guess there is a zip line now...check out the fireworks schedule if you can find it - we were there on a Wed night and the fireworks were amazing. I avoid Marineland.
We stayed in two diff airbnb's in TO (we had 6 nights, 3 in each). One was out towards the Scarborough area, to the east of TO proper, with the lake in the backyard (it was gorgeous). The other was in the York area, a little NW of centre. Both were nice areas with easy access to bus/subway. I'd def plan on parking the car, so look for somewhere with free parking if possible and easy access to the public transport system (bus/tram/subway).
Oh, traffic can be mental on the highway - I know when we were going from Niagara Falls to TO, we stopped in Hamilton for the Canadian Warplane Heritage Museum (for hubby) and when we were heading for TO, our GPS said something like an hour and it took over 2 hrs, so factor in highway holdups.