I've been to Key West half a dozen times, I love it down there.
You can fly to Miami or Ft Lauderdale dirt cheap, and then rent a car for just one day to drive to the Keys. It's a 4 hour drive to Key West from Miami, but it's a beautiful very enjoyable drive.
Once you're at Key West, you can drop your car off, you don't need a car there because it's a small island. Me and my wife just bike or walk everywhere.
You can snorkel, scuba dive, take jet ski tours, or just drink your ass off. It's a fun fun place for around 4-5 nights max.
The other islands are nice to visit to.