Alamo Square --The square is famous for its view of the "painted ladies," a row of impeccably renovated Victorians with downtown San Francisco looming in the background. There are also wonderful views of the Pacific Ocean and Buena Vista. Friendly locals fill the park with their frolicking dogs. Weeknight strolls are ideal for that crucial third date; sunny Sunday afternoons are family time; and early Monday mornings witness the arrival of limber exercisers. (from citysearch) 1101 Fulton
Coit Tower--Atop Telegraph Hill is the 210-foot Coit Tower, built with funds left to the City by philanthropist Lillie Hitchcock Coit for the beautification of San Francisco. The view from atop the east side of the tower spans 360 degrees. Inside, you'll find a history museum and murals that depict working life in 1930s California. From the tower walk down the Greenwich Steps to Montgomery Street. At Montgomery Street, turn right to catch the Filbert Steps which lead to Sansome and Filbert streets, a short distance from the Embarcadero and Fisherman's Wharf. The steep walkways pass beautiful cottages and private gardens. (from sanfrancisco online) 1 Telegraph Hill Blvd
Embarcadero Waterfront--Embarcadero highlights include AT&T Park, the Ferry Building, Rincon Park (Cupid's Span), and a wonderful view of the Bay Bridge. Great photo opportunities. (from about.com)Embaracadero and Broadway
Golden Gate Bridge--Completed in 1937, the Golden Gate Bridge is synonymous with San Francisco and visitors flock to it year-round, simply needing to see it up close. Walking the bridge's span is a popular thing-to-do for tourists and it's absolutely free. Note: the bridge is 1.7 miles long. You can also walk under the bridge by way of Crissy Field.(from about.com) 101 and Lincoln Blvd
Golden Gate Park--This beautiful park is so expansive, you'll need several days to really see it all, but that certainly guarantees you'll see a lot on a one-day visit. From the Japanese Tea Garden to the Conservatory of Flowers to the M.H. de Young Museum, Golden Gate Park offers beauty and education, not to mention a great workout without even realizing it. Along with walking, there's also a bunch of activities to enjoy, including archery, lawn bowling, and a children's playground. (from about.com) HWY 1 and Lincoln Ave
Palace of Fine Arts--The Palace of Fine Arts is a beautiful place to visit. Bring your lunch, have a picnic, and admire the view of a classical Roman-style rotunda and a lovely lake with swans and other adorable waterfowl. A perfect place for some quiet reflection and awesome photos. (Note: the Exploratorium is right next door.)
The Presidio: A former military base, the Presidio is now a public park that's rich with history and activity. Biking and hiking trails call to the outdoor enthusiast, and for those wanting to take a nice walk one sunny afternoon, there's much to see, including more than 500 historic buildings, coastal defense fortifications, a historic airfield, and a pet cemetary. Check out the "Kids on Trails", a self-guided tour that helps kids and adults learn about the Presidio's Ecology Trail.
Walking Tours: For the past 25 years, San Francisco City Guides has been offering free walking tours of the city, led by volunteers who simply "love the city, it's history, lore, and legends." This nonprofit organization consists of more than 200 trained volunteers and offers tours of all points in San Francisco, including Chinatown, North Beach, the Embarcadero, and Haight Ashbury. (Note: As a courtesy and show of appreciation/support, it's always nice to tip the guides at the end of the tour.)
Cable Car Museum--From 1873 till present, learn the full story behind cable cars. You'll also get to see old cable cars and the mechanics behind the current system. http://www.cablecarmuseum.org/
*mostly source is: htttp://gosanfrancisco.about.com/od/touristattractions/p/FreeThingsToDo.htm