I've only ever been here in the summer time for the Tam-Tams jam sessions but there is so much to do around the Mount Royal Park area [plus Fletcher's Field]. Lots of live music in the form of afro-cuban style drum circles, although I've seen live flamenco bands there too, artisanal vendors selling their wares, not to mention plenty of park-space to picnic or toss a frisbee or whatever.
Real hippy-60's-ish kind of vibe but very laid back and nothing like the puritanical parks in Toronto [I'm looking at you Trinity Bellwoods]. Responsible adults are allowed to behave as such without interference from the state, so no worries there. A summertime must-see for tourists and residents alike, so if you're in town between July-August on a Sunday swing by!