Part of the Golden Gate National Recreation Area, wild open spaces with miraculous views. Hiking trails above the Golden Gate are breezy and bracing, and kids like to climb around in the remnants of World War II fortifications. Stop at the visitor center at Field and Bunker Streets for maps to myriad hiking, biking, and equestrian trails and beaches. Besides the Marin Mammal Center (listed below), the main attractions are Rodeo Beach, Rodeo Lagoon, Muir Beach, and the Point Bonita Lighthouse.
The lighthouse is perched on a bit of rock at the entrance to the Golden Gate, with incredible views and a (slightly) swaying footbridge over crashing waves; walk down and
back on your own and get the history from the ranger in the tiny visitor center, or opt for the guided walk, which takes (it seems) forever. Precipitous clifftop trails near here are not for little kids.
I've never seen anything more beautiful. Between the rolling hills as you approach the headlands to the pebbly shore and spectacular views of the cliffs, this has to be the single most gorgeous place I've been. Not good with little kids, but older ones will enjoy watching the waves crashing on the cliffs.