I did this. Flew into Miami, then stayed at fort lauderdale and key west. Def need a car. I loved it there.Hire a car / Motorbike and take the drive down to Keywest and spend either a Friday or Saturday night down their.
Its like a posh Blackpool with everyone trashed around half 11, mucho fun Not much else to see really, the Little Whitehouse, Hemingway's house and old battleship you can walk around and of course a key lime pie or Ice cream.