Montreal is home to numerous shopping options and not only in downtown. However, if you are downtown and on Sainte-Catherine Street, you will find many many opportunities: Department stores, shopping malls, independent stores, known brand names and unique boutiques for home décor or fashion. There are a lot of possibilities.
I’m personally not a shopper but the malls can be surprising and some present beautiful features in addition to offering air conditioning in the summer and warmth in winter. For a lunch break, food courts can please different tastes under the same roof.
There are some interesting places to shop on Sainte-Catherine from Peel Street. You can find the Cours Mont-Royal where a Barbie exhibit is showcased (not a doll museum; it is actually an elegant exhibition presenting designer outfits in a refined section of the mall with chandeliers and a fountain). As you walk to the east, you will come across the Place Montreal Trust with its indoor fountain and glass elevator. Place Ville-Marie offers an observation deck and Promenade de la Cathedrale is right below the Christ Church Cathedral which is nice with the memorial garden to Raoul Wallenberg. In front is the nice Birks store for refined jewelry, La Baie d’Hudson is an historic independent department store and the Complexe Desjardins has a big agora where events sometimes occur. This is just to name a few. Please note that many shopping malls are interconnected via the underground tunnel and metro station.