Top of the list must be the most famous attraction of San Francisco, the Golden Gate Bridge. This 1.7 mile suspension bridge was opened in 1937 and carries both the U.S. Route 101 and California State Route 1 between San Francisco and Marin County.
A good way of seeing the bridge and the stunning views of the mainland from it, take a stroll across this beautiful structure. Just bear in mind that pedestrians are only allowed on it at certain times, so ensure you check this out before you go. A popular way to complete this walk is to stroll across the 1.7 mile stretch and then get a bus back to your original side or to walk half way and then head back on yourself.
Nearby is the Golden Gate Park, a 1,017 acre urban park where there are lakes to laze by or if you're a fan of flora, the park is home to a huge range of flowers and trees. There is even a Bison paddock in the park, which is open to the public for viewing. If none of this appeals to you, there are a catalogue of (usually free) events that take place in the park including live music, cooking lessons and interesting talks.
If you're travelling out between January and July, why not go and see the sea lions at Pier 39. Around 1300 come to rest at this location and it is certainly a sight.
For a beautiful view over San Francisco, head to Twin Peaks, which stands over 900 feet above sea level. However, best to get a taxi to the top as it's a steep walk!