The Cheap Bastard's Guide to San Francisco: Secrets of Living the Good Life--for Free! (Cheap Bastard)

The book contains a number of cheap/free ideas for those visiting the city, including terrific suggestions for shopping. Definitely recommended for both parents and singles.
This book is packed with great facts and activity suggestions for both locals and tourists, and for those, like me, who go to SF often as a day trip.

It covers pretty much every interest a visitor might have. Plus it's a slim volume, reasonable to stick in your backpack.