If El Morro is open late enough to give you enough time to visit, I would do that. It's the fort you always see pictured in PR promos. We walked to it because we had plenty of time and we were basically ambling with some friends of ours that live in PR, but it is a quite a distance and I would probably recommend taking a taxi if you're pressed for time. It's a self-guided tour and probably requires at least 90 minutes to do it justice. It's a great spot for photos though. You can also do the Bacardi rum distillery tour, but again you will need to check the hours. And finally, there's a very nice hotel (I think it's the Wyndham, or something like that) that is just a block or two from where the ship docks and they have a casino. The payouts were pretty bad since they are the only casino in that area. If you are going to be there during the evening hours I would take a taxi to the Condado area (still part of San Juan) and visit the casinos on the strip there. Since there is more competition, the payouts will probably be better.