We typically see the fat tire bike craze outside of California because it's thought to be a snow bike. It's not. Fat tire bikes are a blast on sand and regular old fashioned dirt. Out here around San Francisco Bay, there's tons of muddy gravel marshy places to ride too. Just be sensitive to the local bay habitat.
On a recent ride, we saw fat tire bikes around Half Moon Bay and the deep sandy shore is a great place for a fat tire bike. You'll see not many bikes on the beach and plenty of wild life, both living and dead. Recently, we've had our share of whale carcass for instance. On my ride I noticed a starving young seal stranded on the beach. It's nature and the local waters aren't very healthy at the moment. Food supplies are down and seals are hungry so they go up on shore out of exhaustion.
If you want to get off the sand, you can ride for miles on some of the costal bluffs too, over looking Mavericks and up towards Pacifica. Wonderful riding with views of the entire Bay Area if you can make it to the top.