If you have time and have any interest, Old San Juan is very interesting. The little alleys and El Morro are very picturesque. And, there are cats all over the place, as I recall.
If you are on a Carnival cruise, your ship docks within walking distance of Old San Juan. The other pier (Pan American?) is a cab ride from everything.
We went a night early and stayed in the Condado area. It's more modern, of course. The beach behind the Marriott was beautiful. And, if you like pizza, Danny's is great!