It depends on what you like. I would suggest Puerto Rico, St. Lucia or St. Maarten. They each are friendly, have lots to do, great restaurants and nightlife. Puerto Rico and St. Maarten also have great shopping.
That's prime hurricane season, so you may want to consider travel insurance for your trip. The ABC islands [Aruba, Bonaire and Curacao] are less-prone to hurricanes than the other islands.