I have tried to book a San Juan trip on multiple occasions. The rewards flights to these locations seem to be severely limited.
You can check the Airtran site to see when such Rewards are available. I suspect your options will be very limited.
I just checked on the Airtran Website and outgoing flights are available on 3/10, 3/12, 3/17, 3/19 and 3/28. return Flights are available on 3/19, 3/28 and 4/3.
Good Luck.