Parc du Mont-Royal is like Central Park in Manhattan but has more of a true forest sanctuary vibe.
There's so many great trails to walk and see animals hanging out in the foliage. Numerous picturesque views of the city as well. It's a little slice of natural paradise in a big city.
Of course, no visit is complete without the required photo op at the top with the Montreal skyline in the background.
Parking is reasonable: $3/hour but only $10/24 hours max. Personally, we like to drive up and then walk down into town and eat out way through the city, then burn it all off by trekking our way back, all uphill.
It's a favorite locale to visit when in Montreal and well worth it.