I second Patrick's GGNRA choice, it was a gift of political grit and vision. It all could have been lost to pavement for which the region would be the worse for. Plus it abuts Point Reyes National Seashore, Mnt. Tamalpais & Samuel P. Taylor State Parks amongst others, the public lands of the Marin Water District and other open spaces.
For folks down the Peninsula a similar patchwork of federal, state, regional, and local open spaces provides places to walk, bike, nature watch, and enjoy. Same with the East Bay Regional Parks. We live in a place blessed with a unique climate, geography and flora and fauna that are remarkably diverse. If you’d like to become more aware of such things I can recommend looking at the web site of Bay Nature: http://www.baynature.com/
San Francisco Bay/estuary is a wilderness that provides bounty, joy, beauty and challenge. It beckons and tantalizes. Whether one views it from its shoreline, commutes on a ferry, sails a boat, paddles or rows a small craft you cannot be but be moved by its moods and vistas. Or like Patrick and those South Enders/Dolphin club-ers and other water men/woman who swim, surf, or para-sail the Bay you know & understand the rhythm of tide, currents, wind, and the cold choppy salt water that it encompasses.