If you are unable to get ferry tickets, you can always take a flight to Vieques or Culebra. Again, the choices are limited but the prices are reasonable and if you have a rental booked already...you have no choice!
Here are some options:
Vieques Air Link - 787 534 4221
Cape Air - 1-800-227-3247
Air Flamenco - 787-724-1105
SOME EVENTS TAKING PLACE IN JUNE
Guanica Crab Festival - June 14-16
Festival de San Juan Bautista - June 24th