San Francisco may be my favorite city. The first time I visited, I walk so much I wore out a pair of shoes. While hilly (an understatement), it is a city that must be walked to be truly enjoyed. Of course, your visit would not be complete without a cable ride, but hiking through the neighborhoods brings the greatest enjoyment.
Take your camera, backpack and comfortable shoes and explore. From the Haight, to the Legion of Honor, to the Monestary of Perpetual Adoration, down/up Lombard, through the Wharf, China Town, to the de Young and the tea gardens and shopping in Union Square you will have a wonderful experience taking an urban hike.