Montreal itself is a beautiful city to visit in the summer. The architecture, the wide boulevards and the old city make walking about on a sunny day a true pleasure. Somehow, Montreal seems much more vibrant than at any other time I had visited, with the possible exception of Expo '67.
If you go, be sure to add on another day and visit the textile shops on St. Hubert. There are several, but C&M Textiles and Goodman Textiles are two of note.