Although there is plenty to do and see in San Francisco (and its fair neighbors to the north - Marin and Sonoma -merit a lot of attention too), the newest tunnel in California has made the South Bay just as accessible - and without a bridge toll. For over fifty years, residents on both ends of infamous "Devil's Slide" crossed their fingers while driving over this scenic but treacherous stretch, especially during the winter when heavy rains sent slides of mud and boulders across the highway, sometimes closing it for days.
However, five years of tough tunneling and millions of dollars later, Devil's Slide has been circumvented by one-way tunnels bored right through the hillsides. These two 4,100-foot tunnels, together the longest tunnel by far in California, is an engineering marvel and alone worth the trip.
Devil's Slide Tunnel — Photo courtesy of Wikimedia
If you have the time, you can fly through the tunnels and take scenic Highway One down to Half Moon Bay, even all the way to Santa Cruz, just a few hours away. But if you need to get down and back in a day, maybe hoping to catch some San Francisco nightlife in Chinatown or North Beach or the Castro to cap your day off right, then make your first and only stop the historic and beautiful Montara Beach Hostel and Lighthouse. 
Montara Hostel and Lighthouse — Photo courtesy of Tom Molanphy
As the name implies, this is a working hostel, and definitely a low-budget option that's near enough to San Francisco. But the hostel grounds are open to visitors during the day, so pack a lunch and enjoy the fantastic views of the ocean from their picnic area. And don't worry if you forgot to pack lunch - the town of Montara is just across the street and offers plenty of options.
Public parking is available onsite at no charge - the hostel just asks that visitors sign in at the desk. Although visitors aren't allowed to enter the lighthouse, it's a wonderful sight; sitting right on the cliff, a grand remembrance of old seafaring days.
Montara Lighthouse — Photo courtesy of Tom Molanphy
It's hard to find a quieter, more intimate view of the Pacific. And don't forget to bring your binoculars for outstanding whale and seabird viewing. After lunch, walk through the hostel's native garden and trek on down to their secluded, but open-to-the-public beach.
Beach at Montara Hostel — Photo courtesy of Tom Molanphy
If you gave yourself an early enough start, and the quick stop and small hike wasn't enough for you, you can hop back into the car and continue your exploration of the peninsula by driving farther down Highway 101. Locals say that on a crisp fall day or a fine spring morning, nothing beats a drive down this iconic California highway. And with the addition of the Devil's Slide Tunnel, wet winter weather shouldn't stop you from at least getting as far as Montara Hostel and Lighthouse, a true gem of the southern peninsula.