Sadly, it is difficult to maneuver around without a car. There are buses whose schedules can be found on the internet, but I don't think they are very convenienet. Can't really walk anywhere. The Tren Urbano, the new metro system puerto rico is building, is supposed to start up soon, but there have been delays.
However, it is good to know that when driving around late at night one does not have to stop at the red lights, as a precautionary measure against late-night, empty-road dangers. This is a possibility after 11:30 or midnight (not sure exactly when, but around there). Anyway, this doesn't mean you can just eat the light-- make your approach, be alert, and proceed with caution. Traffic accidents are horribly unpleasant, and if you wreck your car then you have no means of transportation. And that is very sad.