Leo707, welcome aboard.
As for your question : What is your objectives of type stuff to chase ? Eg.: jewelry ? Old coins ? relics ? nuggets ? Etc....
If it's jewelry, then any swimming beach (fresh or salt water) are your best odds .
If it's nuggets, then Sonoma is in the wrong part of CA for that.
If it's "old coins" (new coins are boring after all, eh ?
) or relicky type persuits, then you're going to be in a bit of a pickle asking that question. It would be like asking a trout fisherman: "Where's your trophy trout holes on the creek ?" They might clam up. Unfortunately, old coins are not a renewing resource, as is jewelry on the beach (or never ending like nuggets or whatever).
There is a monthly club meeting in Santa Rosa. If you attended there, and paid close attention to who's bringing in oldies to show & tell, you *might* be able to befriend them, buddy up, etc.... I say "might", because even then/there, some are tight-lipped if they've got a good site. Eg.: defunct resort spots, stage stops, old camp grounds, etc....
You just have to research.
I will say this: Your part of CA has a LONG history of m'ding. Hunters plying those parks since the 1960s even. I've tried a few old parks in your area (Napa, for instance), and you have to be very skilled to pull any more silver out of them. If all else fails, you can knock on doors and do the yards of old homes. Or watch for old town urban demolition tearouts (old town sidewalk tearouts, or building teardowns, for instance. And get in there before they dig too deep, and/or before fill-dirt is added.