First thing I do is go to the ship and have lunch, then I go into Old San Juan.
I walk along the cobble stone streets by the waters edge, visit a few shops, look at a few vender stands, smell the flowers, look at the fountains and the scultpures, stop and have a drink at the El Convento, go into the old Church across the street and say a prayer to say thanks for being here, go around the block to a beautiful plaza that has a foutain and statues, look at the small children playing and chasing the pigeons and then I buy some soda to take back to the ship.
You could walk to El Morro, go to the rum factory or gamble at the Sheraton but what I do is enough for me and helps set my mind for the cruise to be.