If you plan to rent a car, it is *critical* that you print out a copy of your current car coverage and bring it with you to the rental agency. Otherwise they will force you to pay for rental insurance, which can average at $14 per day for the lowest coverage.  Also would suggest that you pay the rental company for the highway toll system so you can drive through the few pay-roads that they have.

Finally, would suggest that you bring some mosquito repellent and make sure to use it at all times. Puerto Rico is expected to be affected by the Zika virus. There are rainy seasons that can make it worse, so you might want to consider the seasons when traveling.