This Saturday is California Coastal Clean-up Day, which runs from 9 a.m. to noon. There are many participating sites, and you should contact the coordinator in advance. Find out more here.
A sample of sites:
Marin: Rodeo Beach - sign up here.
San Francisco: East side or West side - sign up here
San Mateo County: sites in Belmont, Pescadero, Redwood City, San Mateo, South San Francisco and many more - sign up here.
Contra Costa County: sign up here