On a nice day, you should head down to Old Montreal and walk on the cobblestoned streets. It's a great place for pictures, for souvenir shopping, for eating and for just people watching.
When you're done with this area, head over to the Old Port area along the water and do some more walking along this area. If you're tired of walking, you can always pay for: a horse-drawn carriage ride, a double/triple/quadruple seated bike or go to the water and rent a paddle boat.
A lovely area that even the locals appreciate, for we saw 4 pairs of newlyweds getting their photos taken around the area.