MONTREAL! MY LOVE! Ahem.
St. Catherine's is fun, but so is St. Laurent (which also happens to have Montreal's MAC pro store, but that's not the point). The biodome makes me happy, especially the penguins. Out that was is also the Botanical Gardens, which are beautiful. There are, like, ten green houses and a huge amount of space outdoors, too. The insectarium is fun, but not nearly as cool as the biodome and gardens.
Basilique Notre Dame is beautiful, and it's in Old Montreal. Whether you go to see the basilique or not, you really must visit the old port. It's GORGEOUS. And, if you have the money for a horse and carriage ride, do it! It's a fun tour. There's even one street in the old port that still has gas street lights!
Montreal's "underground city" is lovely. The subway system (known as the metro) is lovely and kept quite clean, as well as having underground, well, everything along with it.
You should climb Mount Royal. It's gorgeous. Up at the top, there's a lookout built in where you can look out across all of Montreal. It's particularly sensational on a clear night, when you can see all of the lights.
Part way up Mount Royal is a park, and on Sunday mornings in the summer season there are the Tam Tams held there. Basically, there are a bunch of people with drums and some funky stuff to sell. Don't go if you're adverse to the smell of marijuana, though.
To eat, you should make sure you get a proper smoked meat sandwich. You simply should. I also love walking down the street and ducking into a shop for a fresh bagel and walking along chewing on that, but it's no smoked meat sandwich.
My beloved McGill University is in Montreal, but it's not really something I think you need to go see.
There are a number of museums about, including one on campus, though.
There's just... so much. There are so many fantastic sections of the city reflecting different cultures and ohhh the food...
I love that city.