The largest of Golden Gate Park's man-made lakes, Stow is a hub of activity including hiking, bicycling and, formerly, boating. The boathouse rentals (boat and bike) are currently closed, and catalyst of an ongoing battle.
The lake surrounds Strawberry Peak a short but fairly steep hike with nice views on top (a free and outdoor version of the de Young observation deck) and a good place to spot wildlife - fish, basking turtles, endless human-training squirrels and all kinds of birds... including a variety of ducks :)
Address: Stow Lake, Golden Gate Park, SF