Have room service for breakfast, so you can get off the ship as early as possible, Walk to the left of the pier and get a pocket map (on baord ship or) at the Pink Information building near the LaPrincessa park. Walk all the way through this beautiful park through the Old San Juan gates up to The fortress El Morro. Once you explore this beautiful renovated fortress, it is downhill from there literally. There are many cathedrals to check out as well as historic buildings and by ways. Stop into the El Convento Hotel for a peak which is across from the San Juan Cathedral. By this time shops on Calle Forteleza should be open. Check out the Butterfly shop & cafe as well as the Puerto Rico Art Association shop for beautiful one of a kind keepsakes or just a look & see. Many of the open plaza's often have entertainment on Saturday, so this may be an unexpected treat for you during your walk. Get back to the ship for a late lunch on board for sail away.
Bon Voyage.