The Baylands is the largest remaining tract of undisturbed marshland in the San Francisco Bay, a flat, wide-open landscape that feels surprisingly remote given where it sits. Trails run out through the tule reeds along the bay shore, where great blue herons, avocets, and willets are regular company. The Lucy Evans Baylands Nature Interpretive Center, a modest building on stilts over the marsh, has exhibits on the local ecology and is staffed by knowledgeable volunteers on weekends.
The main entrance is at the end of Embarcadero Road, where there's a small harbor with rental kayaks available seasonally. At low tide the flats extend far out into the bay; at high tide the water comes right up to the levee trails. Either way it's a good place to clear your head.