Totally possible, and the ferry is really a delightful trip over to Sausalito. And you really don't want to rent a car unless you have to. Everything you need to know is here:
http://goldengateferry.org/schedules/Sausalito.php
http://www.blueandgoldfleet.com/Ferr...103110no51.cfm
Note there are two lines with two different schedules. Also be advised Sausalito is way more manageable on a weekday. The weekends there, especially in Sept/Oct (our best weather) are thronged with tourists from all over the world.