Wow Noell, you must be really getting excited at this point...You can try to get checked on board as soon as you land and get to the cruise port and then go back out, or you could check your luggage somewhere and spend some time touring. I'm not sure what you have made for transportation from a/p to cruiseport.
You would certainly have time to see some of the Old San Juan area. They have an open tram
that you can get on and off along the way and see lots of grea things as you ride of get off for a short time and get the next one and continue. You will go by both of the forts if you do this..You can go to the San Juan site and check things out. Have fun there and on your cruise. We were there for two nites prior to our cruise there in Spring 2009.