Humber Bay Park is a nifty little collection of peninsulas spreading out into Lake Ontario, just southwest of the Humber Bridge, at the base of a spate of new condo developments. It's a bit of a hidden gem for that reason, and the new buildings actually shield it even more from the noise and traffic of the Lakeshore and the QEW.
It's very scenic, since it has a high ratio of waterfront-to-park; nevertheless, its area is small, so anything requiring more space than picnicking or reading in the shade is pretty much out. It's accessible from the waterfront bike trail that goes past it, and is itself pretty fun to go though on a mountain bike.