This is the fanciest, biggest, most glorious farmer's market in the city. I find it hard to stop here for less than an hour, and you're sure to see a famous chef doing the shopping for that evening's upscale dinner crafted from fresh organic local seasonal heirloom (did I mention expensive?) produce. With ~40 produce stands, there's a wide range, from humble and affordable to super-gourmet, and if it's in season in California you are certain to find it here. There are also ~10 prepared food stands for snacking as you browse, along with a handful of miscellany, such as bakeries, fresh pasta, and meat.
The view of the bay is also a highlight, as is the historic ferry building itself around which the market is clustered. Inside the building are permanent stands with additional options for produce and nibbles, as well as Sur La Table if you need extra kitchen implements for your creation, so if you're willing to spend $$ it's a one-stop shop.
It's easily accessible by ferry or public transit on land; I wouldn't recommend the hassle of parking if you can avoid it although they do advertise valet service. The market also provides conveniences such as carts for big shoppers, a way to buy tokens all vendors accept with a credit card, and maps of the stands.
It's a don't-miss California experience!
PS. In front of the food market, heading towards Market Street, people have set up an informal crafts market with jewelry, artwork and other handmade items you may also find interesting.