Barbados. Stay on the West Side of the Island.
Reasons why
-It's just freaking beautiful
-Safe yet fun
-awesome water for scruba, sea turtle feeding, boating
-Direct flight from JFK on Jetblue or AA
-You can rent a house if you two wanted
-Not huge with the lower 20s crowd but great for upper 20s, I went 3 years ago when I was 26.
-Exchange rate is 2:1. You double your money because prices stay the same basically. A can of Cola is $1 Bajan so it's like 50 cents American. So you can have real fun at night with the ladies.