SF is a friendly place - but starting today, there is a new hassle for visitors planning to drive a rental car across the Golden Gate Bridge . (What other way to see Muir Woods or get to the gayish nude beach on that side of the bridge?)
The Golden Gate Bridge has eliminated human toll takers (and has no machines) but you still have to pay a toll. People who live here all have electronic pass devices (like the EZ Pass on the east coast). If you drive your own car, and don't have en electronic pass, they mail you a bill. But if you are driving a rental car you have a potential problem. If you cross a bridge and have not paid in advance, some rental car companies are happy to charge you an extra daily fee (from $3 to $10 per rental day) for the privilege of then charging you later for the tolls you did not pay. They apparently will charge this fee for every day of rental starting with your first bridge crossing, no matter how few days you actually cross a bridge. Some rental car companies have no "fee plan", but will send you a bill later for a lot, if you have not made advance arrangements.
http://www.goldengate.org/tolls/rentaltollprograms.php

There are various ways to pay in advance (if you know your rental car license plate number.)
http://www.goldengate.org/tolls/tollpaymentoptions.php#paybyplate
Generally, one can pay a toll in advance at a kiosk - if you can find one. There are supposed to be about 100 in the 400 square mile bay area.
Map: http://batchgeo.com/map/4218650a774cd48c44b07230c6ac0318
Have fun - it can be a new challenge; or just resign yourselves to paying up.