Notre Dame and St. Joseph’s Oratory are the obvious choices in Montreal as already pointed out. Be sure, at the Oratory, to visit the original chapel of St. André, and the beautiful garden of the Stations of the Cross. Quebec City both cathedrals are worth the detour (Catholic and Anglican, the first Anglican cathedral outside of the British Isles), there are also numerous very old churches dating to the 17th Century, including the Catholic cathedral. All are within walking distance in the centre of the historic walled city. Ste-Anne-de-Beaupré shrine is a popular pilgrimage spot, about an hour downriver from Quebec, by car, on the north shore of the St. Lawrence. It’s a huge basilica.
On the way from Quebec City to Montreal, for a complete contrast there is the new Trappist abbey in St-Jean-de-Matha, very modern and impressive architecture:
These used to be the monks of Oka, but they moved when their abbey in Oka became far too big for their dwindling community.
About a 120 km (75 mile) detour from Montreal is the abbey I’m associated with, Saint-Benoît-du-Lac; alternately you can make a triangle from Quebec City, through the abbey, to Montreal or vice-versa. The area of the abbey, the Eastern Townships is very beautiful Appalachian countryside, with a significant anglophone population. At the abbey, Lauds, and Vespers are in Latin Gregorian chant, and the Mass is a mix of Latin/Greek Gregorian chant (for the propers and ordinary) and French plainchant (for the rest). The other hours are in French but with Latin hymn and Latin Marian antiphon at Compline. Lauds at 7:30 am (7 am + Mass on summer Thursdays through labour day), Mass at 11 am except Thursdays (simple recited Mass at 11), Vespers at 5 pm except Thursdays (7 pm Thursdays; Thursday is the monks’ recreation day).
There is another shrine, at Cap-de-la-Madeleine, on the North Shore halfway between Quebec City and Montreal, adjacent to Trois-Rivières (Three Rivers).
https://www.sanctuaire-ndc.ca/en/
If you go there, take the historic “Chemin du Roy” from Quebec City to Montreal, a leisurely drive along the river, instead of the fast autoroute. Lots of history along that route.
Montreal also has Mary, Queen of the World Cathedral, modelled on St. Peter’s in Rome, and St. Patrick’s Basilica (unofficially the “Anglophone” Catholic cathedral), and numerous beautiful churches all over the city.
I could prattle on, just tons of things to see, but these are the major religious ones.
