In the morning, rent a bike near Union square (buy a ferry ticket there), and ride to Ocean Beach through Civic Center, Alamo Square, Haight St, and Golden Gate Park. Then head towards the Legion of Honor Museum, then cross the Golden Gate towards Sausalito, get an ice cream at the marina, and take the ferry at the pier back to SF (you can board your bike, and you'll see Alcatraz from very close). It's going to be your best day in a very long time.