There are many things to see depending on your interests. Old San Juan is in walking distance of the ship. There are several restaurants there as well as the "staples" of The Hard Rock Cafe, and Senior Frogs. The Sheraton across from the pier has a great casino. If you are into history, check out the Fort's (There are 2, both run by the US National parks)
You may want to consult this thread,
where I asked the same questions you are asking
There are some good San Juan ideas there. We had intended on maybe seeing the rain forest, but our flight (that came in the day before the cruise) did not get in until late so there was not enough time.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007