Do a walking/shopping tour of Old San Juan. El Morro, the fort on the west end, is very interesting. Just be sure to wear comfortable walking shoes. There is, of course, the problem of what to do with your luggage.
You are usually docked right in Old San Juan and walking is the mode of transportation or the free trolley. Now RCI and Celebrity sometimes use their newly renovated pier across the bay, which is a taxi ride into Old San Juan less than $10.