Fort Mason is a former U.S. Army coastal post on the bay between the Marina and Aquatic Park, decommissioned in the 1970s and reborn as one of the city's most likeable cultural campuses. Its old piers and warehouses now hold galleries, theatres, a long-running vegetarian restaurant (Greens, with picture-window views of the bay), festivals, and the popular Off the Grid food-truck market on summer Friday evenings.
The grassy upper fort, the Great Meadow, offers wide views over the water to Alcatraz and the Golden Gate and is a fine spot for a picnic. As part of the Golden Gate National Recreation Area it links the Marina waterfront path to Aquatic Park and Fisherman's Wharf beyond, making it a natural stop on a walk along the northern shore. Check what is on before you go — there is usually something.