If you have good weather and the better part of a day to spare, rent a bike and ride across the Golden Gate Bridge. Have lunch in Sausalito and take a ferry back to Fisherman’s Wharf.
There are a batch of bike-rental shops near Fisherman’s Wharf—Blazing Saddles and Bike and Roll have seven outlets between them. Looking at price and equipment, they’re pretty much interchangeable. Maps and written directions are provided for a variety of routes. Helmets and bike locks are included. A basic rental is $26 to $28 per day, but invest a couple of minutes in skimming through one of the free tourist guides; you should find a coupon good for $2 to $4 off.
Generally, the ride is flat and protected from traffic. After some chaos around Fisherman’s Wharf, it’s mostly a protected route through Aquatic Park, Crissy Field, and Fort Point. There’s an uphill to get onto the Golden Gate Bridge and begin the spectacular one-mile ride across on the bike/pedestrian sidewalk.
It’s interesting. Riding on the bike, the bridge seemed solid and steady, but when we got off the bikes halfway across to gawk at the view, everything rattled and shook as cars and trucks hurtled across. We actually felt more secure on the bikes than while walking or standing.
After crossing, it's a short downhill ride to Sausalito and lunch. Total distance to this point is 8 miles. Now, there are options: if you’ve had enough riding, it’s easy enough to catch a ferry back to San Francisco. Otherwise, you could retrace your steps to go back over the bridge or pedal on to Tiburon and grab the ferry to Fisherman’s Wharf there. Either way adds about 8 more miles.
We chose to continue on to Tiburon. It was less comfortable, with more riding on the side of the road while traffic whizzed by, but still, at least half the route was on well-protected bike paths along the bay.
The ferry back to San Francisco was amazingly fast—this wasn’t any tourist sightseeing trip but a functional part of the Bay Area transportation system. It’s got an unusual quirk, however. You buy a ticket upon boarding the ferry, then give it to the ticket-taker while getting off. Unfortunately for one cyclist, there’s a serious breeze on the ferry; his ticket blew out of his pocket and into the bay. As we left, he was still arguing/pleading with the ticket-taker that he shouldn’t have to pay a second time. You’ve been warned: hold onto your ticket!
The best thing about biking is freedom to explore odd bits of the city you encounter in passing: check out the Palace of Arts and Culture, ride out to the end of a marina wharf, spin down side-streets in Sausalito, or go wherever something looks interesting.