Walking around San Francisco is a sure-fire way to get in some good exercise. The hills are steep, and everything seems to be further than anticipated. On the plus side it is fun, and you quite often come across stereotypically 'San Fran' sights.
Like cable cars
Which are even more satisfying when you manage to get a picture of one directly outside the cable car museum
Fisherman's Wharf for all the crab you can possibly eat
Ghirardelli Square for excellent chocolate
And of course the Golden Gate Bridge
Also you occasionally see bizarre things like giant crocodiles made out of bread
And at the end of the day, there's always another hill