Are there car rentals available in San Juan's Luis Muñoz Marín International Airport where the rental lot is on the airport property? The past two times I've rented a car the rental lot is a considerable distance away from the airport and difficult to find when returning the car.
There are no rental car companies that are inside the terminal as such, however there are a few brands that are located on the airport property. All still require catching a shuttle, however the ride is much shorter than to the off-airport locations.
Companies with on-airport locations are :
All other brands are off-airport, mainly in the Isla Verde area which is relatively close to the airport - but most are still a several mile shuttle trip from the airport to/from the rental location.
In addition to considering on-airport locations, there are a number of rental locations around San Juan, mainly in hotels such as the Sheraton. Depending on your exact needs you may find it cheaper to catch a taxi to/from the airport, and instead only rent a car for the exact periods that you need it. This is especially true if you are staying in Old San Juan where parking is an issue.
Not directly related to your question, but when renting a car in San Juan always book well in advance - it is not uncommon for rental locations to sell out, especially around holiday times, and if you haven't pre-booked then you're likely not going to be able to get a car!