Rental rates might be too high, but a few places I know of which could be suitable are:
Sportsman's Club, San Pablo
Veteran's Building, Sonoma
Marin Art and Garden Center, Ross
Some high schools still have a traditional large auditorium (Analy in Sebastopol), but renting one for such an event might be more of a hassle than it's worth.
Depending on how many people want to attend, a large house could also work well. I'm afraid mine is overflowing with horns and people, and has no room to spare.