If you only have a day in the city, go to Mont-Royal park for the views. But if you have more time, I strongly suggest you also visit Laurier Park, just a stone throw's from Laurier metro station.
It's the park Montrealers enjoy the most. Mature trees, beautiful architecture in the surrounding streets and tons of take-away restaurants nearby (not to mention free wifi) make this park incredibly popular from early March to late November, from the first to the last rays of warm sunlight.
It really is a great place for a picnic, for a tanning session or even an impromptu volleyball game.