Go to this page
http://travelandsports.com/ca.htm
and send some email inquiries. I know of at least one Prius driver in San Juan, but honestly the cars are not common there. You may have to put up w/a conventional.
Driving in San Juan is 'competitive' and 'self-assertive'. You have been warned.
If time permits, visits to the Luquillo Mountains and Arecibo Observatory are both good bets. There are 3 bioluminescent bays, including one on Vieques.
Trying to attach a map here to a particularly nice beach a few miles east of the airport. There is a parking lot inside my red circle. Then find the trail through the bushes to the northeast, starting at a rusted car hulk. Some adventure eh?
General tourism info at
http://welcome.topuertorico.org/index.shtml
Barrilito three stars rum, if you're so inclined, and for the full experience (and best price) visit the folksy factory. You may bring me back a bottle if you wish.
DAS