Go see El Morro, and walk up the hill that Teddy Roosevelt rode up. Go to Barrachina's and have a pina colada. If you have time, do not miss the rain forest. Stay in a hotel on the beach. Best restaurant in town, Marmalade, is fantastic, and the owners come to chat with you. $75 per person and worth every cent.
If you have missed Diamonds International or Little Switzerland in other Caribbean ports, this is your chance. Actually, there is nothing worth buying that you can't get from a reputable source on the net. Easy walk to and from the ship.