Culebra and Vieques Puerto Rico have 2 car rental locations, 1 on each island, from Avis. These 2 are smaller islands East of Puerto Rico. You can fly from San Juan or take a ferry from Fajardo to get there.
Here are some sites discussing flying to Culebra or Vieques compared to taking the ferry.
The ferry ride is $2.25 each way or $4.50 round trip. Plane tickets from San Juan are about $105 round trip.
The ferry ride is $2 each way or $4 round trip and takes about 1 hour 15 minutes. Taking your car on the ferry is $26 each way and takes about 2 hours. Plane tickets from San Juan are about $170 round trip and 30 minutes long each way.
Calle Escudero y P Marquez 37
Culebra PR 00775
State Rd 200 KM5 H1
Vieques PR 00765
M-F 9-12, 1-5, SAT 8-12, 1-4, SUN 9-1