Well if you are going to San Juan
my recomendation will be to stay on the old San Juan, renting a car is not an option.
San Juan can get very difficult during the day, even in the highway you will have traffic jams that can take you for ever to get back to the ship. El Yunke (rain forest) is far away from the port like a 45 minutes car drive and considering the traffic problems and the situation that you have never drove a car in Puerto Rico, believe me it can get very difficult even for me, I don't recommend it.
Now what I recomend is to stay in old San Juan maybe you can visit "El Morro" is not far from the ship, not a lot of $$$. You will not have any trouble getting there even walking if you are up to it, not my case, but there are also a few trolleys in port that can take you there.
Also old San Juan have a lot of places to visit:
* "La capilla del cristo" this is the resting place of Juan Ponce de Leon the historic person that was looking for the fountain of youth.
* "The home of la Pina Colada" this is the place where literally the pina Colada was invented maybe not exxactly the place but the owner is the one that invented the drink. Yes the Pina colada was invented in Puerto Rico...
* "El morro" an spanish fort in good conditions and a good photo oportunity.
* "Bacardi" here I will recomend to book directly from the ship excursions and besides is our rum destilery so you will get good prices in the rum and yes is a strong rum be careful with it.
I have live all my life in this island you can send me a message if you need any info...