There are many ways to get out into the great outdoors in the Bay Area with children, and GGMG moms are recommending the following resources: 

Easy Bay Area Hikes
GGMG's Laure Latham's article was published in the February 2007 GGMG newsletter.

Sweeney Ridge, GGNRA, San Bruno
Take 280 South and exit Sneath Lane and drive west, following Sneath Lane, about 2 miles to the trailhead at the end of the road.  There is a paved path to the top of the ridge, with views (depending on the fog) of the Bay and the Pacific. It's about 1.4 miles to the top, with a 500 foot elevation.  There's much more hiking in the area, as this site shows.   Note: the gate is too narrow for strollers, so don't load up in the parking lot--get through the gate first.