On a friend's recommendation, my fiancé and I rented bicycles from Blazing Saddles at Pier 39 Fisherman's Wharf and cycled to Sausalito. Rental costs were expensive, even after the $5 coupon found in the many free local guidebooks. However, the bikes were good and came fully equipped with locks, pouches, and maps. I believe there are cheaper shops around if you don't mind hunting for them.
We decided to take our time, spending a day cycling and making many stops along the way to take pictures and visit some shops. The journey to Sausalito takes you along the bay, across Golden Gate Bridge, and into Sausalito. The bike trails were easy to follow and clearly marked. The views were fantastic throughout, as they also were on the ferry back. Along the way, we saw some couples struggling along on tandem bikes. While this option is cheaper and sounds romantic, I'd recommend against it, as you lose maneuverability. Also, parts of the trail can get steep, which means bad news for us masculine fellas providing the pedal power.