Just to give you a couple more ideas, consider Belize and Curacao. Ambergris caye is reached via small plane or ferry from Belize city. I've been to the backpackers hotel there visiting. Clean rooms. Nice bar and it's cheap. Ambergris caye in general is a pretty cool place. Take the ferry from Ambergris to Caye Caulker and you're back in the '60s lol
Curacao is bigger but can offer cheap flights to Bonaire. Diving/snorkeling there is awesome. It's a bit more laid back. Not so touristy. Both can be reached on AA via Miami.