One great way to spend weekend mornings is to check out the freshest produce, delivered right into your town by the local Northern California farmers. Not only do you get to pick from the best fruits and vegetables the area has to offer, but you also help your dollars go back into the local community. The Pacific Coast Farmers Market Association publishes a detailed calendar to where you can find the local farmers markets (and what vendors will be there) in your area and the SFGate publishes a more comprehensive list of cities, but no vendor information.
Here's a sampling of the farmers markets in the cities across Silicon Valley and the rest of the Bay Area; many are open year-round and some of them are open during the weekdays too.
List of Many Farmers Markets in Silicon Valley
You can find their hours of operation and details on how to get there from the Pacific Coast Farmers Market Association Santa Clara list
Since the PCFMA is interested in promoting agriculture, they list the farmers markets that are predominantly about selling produce as opposed to arts and crafts. Since that's a pretty good indication of whether the market is more about food or being a street fair, you can choose one close by that matches your tastes.
KQED provides more links about Bay Area markets and some background about organic products and sustainable agriculture.
And If You're Around Mountain View...
There is also the Milk Pail Market which provides fresh produce at prices that are considerably less than the Safeway across the street or the Albertsons nearby. They're open daily and I do a lot of my personal produce shopping there because I prefer to shop in the evenings. (And nobody paid me to say that! Or anything else on this site for that matter...)
While Milk Pail compliments this selection with a wide range of cheeses and also have some gourmet pre-cut meats you can choose from, the Dittmer's Gourmet Meats and Wurst-Haus on San Antonio just a block away is truly the place to go for that. Their list of smoked meats includes an award-winning smoked chicken and a bacon that makes you question what you thought bacon was. And after you go, everyone will know you were there because your clothes will come out slightly --- but authentically --- smoked!