DMB: You should be psyched - you're going on a cruise! Regarding your birthday plans, there are lots of cool bars in Old San Juan but I'd advise caution - don't get too "happy", especially at night, and go in a group of more than 2.
I've had great times in San Juan and, on the other hand, once saw a crewmember have his face sliced open in a bar, for no apparent reason. You can bet there was a grand exodus of passengers from that place and a quick return to the ship.
Just use common sense and be aware of your surroundings as you would be in any city. Have fun!