There are lots of little keys dotted around the Atlantic Ocean/Gulf of Mexico, but this string is connected by the Overseas Highway:
Plantation Key
Upper Matecumbe Key (Islamorada)
Indian Key (Islamorada)
Lower Matecumbe Key (Islamorada)
Long Key and Fiesta Key
Conch Key
Duck Key
Grassy Key (Marathon)
Crawl Key (Marathon)
Vaca Key (Marathon)
Boot Key (Marathon)
Pigeon Key (Marathon)
(Seven Mile Bridge)
Bahia Honda Key
Spanish Harbor Keys
Big Pine Key (and No Name Key)
Little Torch Key
Summerland Key
Cudjoe Key
Saddlebunch Keys
Boca Chica Key