San Francisco is a walking paradise for those who don't mind a hill here and there!
The Lands End trail is epic! Golden Gate Park hold many charms and finishes with the expansively walkable Ocean Beach.
The eastern and northern waterfront offers no end of basically flat walking from AT&T Park, along the Embarcadero, up to Fisherman's Wharf, the Maritime Museum, Fort Mason, Crissy Field and finally Fort Point. From there, the Presidio and the Golden Gate Bridge are a sizable hill up, with Baker Beach (to the west) and Sausalito (across the bridge) at the bottom. The Blue and Gold Ferries can be a good option for getting back Downtown and/or to Fisherman's Wharf without retracing the journey.
For more urban walks, the Mission Murals can be fun, and SF City Guides provide a route and cultural edification :)
Beyond SF, Marin County has endless trails... Mt Tamalpais, Muir Woods, and Point Reyes are especially memorable. Closer to home, Angel Island has some great trails and views.
