Answer: My scooter!
Why?: Three reasons--it gets 70 miles per gallon, is easy (and free) to park on campus, and it's fun.
Moving to L.A., my husband and I debated whether or not we needed to bring the 2 cars we already owned. We decided to try out being a single-car family.
My first commute mode was by bus—convenient and cheap, but for off-peak rides home, definitely made the day longer. Then I noticed that many students get around Westwood on scooter.  After researching low-traffic routes to and from campus, I decided to sign up for a motorcycle class and bought my first scooter—a used Vespa, for around $3K.
It's a low carbon footprint mode of travel, takes up very little parking space (which makes the transportation planners in the Luskin School very happy) and is very economical.  In addition to the cost of the scooter, I've invested about $250 for the class, $80 a year for insurance, and about $400 in protective gear.
The students at UCLA really had a good thing figured out—glad I caught on!