The Golden Gate Bridge is usually #1 on visitor’s lists, and is possibly the most recognizable San Francisco landmark. You can see it from many places, and even stroll across it to Marin County.
Here is some good information about the bridge.
At the foot of the bridge on the San Francisco side, under the arch is Fort Point.
Definitely worth an outing. If you have time, walking or riding a bicycle across the bridge will bring you eventually to Sausalito, which is a terrific place to visit.