The Golden Gate Bridge is not to be missed! It's best up close and personal. The bridge is a bit less than 2 miles across - an excellent hike or bike ride. Be sure to wear warm clothes, as it is often windy and/or cold.
On the San Francisco side, the Presidio and Crissy Field are excellent locales to explore and get views of the bridge.
A stunning view often rewards those who venture up the Marin Headlands, follow the ridge to the left on the Marin side of the bridge. Down the hill is Sausalito, including ferry service back to San Francisco.
Wikipedia: http://en.wikipedia.org/wiki/Golden_Gate_Bridge