Above the rapids, sheltered and parking is usually empty (before lunch).
From there, it’s 40 minutes to an hour (depending on wind and waves) across to Oka Beach. This is probably the nicest beach in the Montreal area. Even on weekends, it’s virtually deserted until lunch time. Bring cash, it’s possible the Park Police will patrol and ask you to pay the entrance fee if you stay on the beach.
So not strictly Montreal area, but worth the trip.
Launched the kayak at Ivy Lea campground just west of Rockport. Currents can be strong, but manageable going upstream towards Gananoque. Water visibility is spectacular, Gliding over the water the bottom can be clearly visible 20 or 30 feet below.