My wife and I visited Puerto Rico in October of 2010. The first place we visited was Old San Juan. The fortress of El Morro is not to be missed. You could literally spend a half day there alone.
The rest of Old San Juan is full of historic buildings, cobblestone streets, street vendors offering arts and crafts, and great restaurants.
We spent a full day in Old San Juan and enjoyed just walking around taking in the sights, shopping, and eating at a Cuban-style restaurant .
An arts and crafts fair near the cruise ship docks had great offerings.