Here is a quick itinerary from the old port that is not too long (~30km) and using cycleways all along, for folks who want a relaxing ride.
Apologies for using google maps, I could not find a matching trace
It follows the Canal Lachine, which was a very important waterway during the development of Montreal.
You can stop at the Atwater market, where you will find plenty of fresh produce, cheeses (should you need to buy some for the C&W party)
At the end of the canal is the Jacques Cartier parc, perfect place for a picnic. You'll see many families on the barbecue if the weather is nice. At the entrance of the parc is an ice cream vendor IIRC.
This is not a city tour, more of an activity Montrealers may do themselves.
If you want to cut the ride, you can go to the Lionel Groulx station (green line), and rent bikes from MyBicyclette*: