We are so lucky to live walking distance from this park (in fact, it was part of the reason we chose to live in Ahuntsic).
In summer and fall, there are lovely walking/running/biking trails beside a beautiful river. Near sunset, look out for the great blue herons and the night herons (bihoreaux) near the old moulin. It's also wonderful to head up near the dam and find a bench to watch the sun set over the river. There are lots of picnic tables and grassy spots to spread your picnic blanket.
In winter there are groomed trails for cross-country skiing and snowshoeing trails. Check out the events that happen at the maison du parc -- we recently attended a moonlight ski which ended with a campfire and hot chocolate. Magical!