Good luck. :P The thing about Toronto is that when architecture gets old, it gets torn down and something new and ugly gets put in its place. It's like Logan's Run for buildings.
You might find something in the Roundhouse Park. It's just south of the MTCC across the railroad tracks and you can take the indoor Path to get across and outside. The Steam Whistle Brewery and the Railway Heritage Centre are there.
Just north of Union Station, across Front Street, is the Fairmont Royal York Hotel. Pretty exteriors but maybe not anything at street level outside that's good for photography. (It might just look like a plain stone wall.)
From there, go one more short block east and go north on Bay Street to the entrance of BCE Place. There are some neat old stone building facades inside there that have been preserved basically by building a glass atrium around them.
If you walk as far again from MTCC as you already have by that point, continuing east along Front Street, you'll come to St. Lawrence Market. This is an active marketplace where you can buy food and stuff, and it'll be super busy on weekends and will probably have an outdoor flea market too. Most of the external architecture you'll see is 1890s-1900s. Fun fact for history nerds: there's been a market there for as long as there's been a city of Toronto, and before European colonization it was probably a trading and meeting place for as long as there have been humans in the area.