You could just ask 4WheelBob where he goes, but Bob gave up on “accessible” trails years ago. Web sites of the various park districts are all over the map on telling folks which way to point their chairs: Midpeninsula Open Space District’s page is wonderful, Santa Clara County’s is invisible (if it’s in there, I couldn’t find it), East Bay Regional’s is in the middle, for instance.
The California Coastal Commission has a free book outlining all the trails near the water — you can download the book or order it by e-mailing firstname.lastname@example.org or calling (510) 286-1015. Link here.
Trail Center, based in Palo Alto, has an excellent page describing accessible trails from San Francisco down the Peninsula to Santa Cruz.
BA Hiker has a page of all-access and easy hikes (thanks, Jane!)
And, last but not least (by a long shot), Ann Sieck profiles a bunch of North Bay and Marin trails at wheelchairtrails.net (she’s also wrote one of those Bay Nature articles.) Here’s a list of her hikes indexed by city.
I like Bob’s outlook on accessibility (which he will no doubt expand upon): It’s not reasonable to expect all trails to be made accessible to the disabled, nor even most of them, but there certainly could be more accommodation than there is now. Main thing: you don’t have to be a fourteener-climbing borderline madman to get a taste of the wilds from your chair.