The 140-mile drive from the southern edge of Miami [1] to downtown Key West [2] only takes about three hours, so it’s quite possible to “see” the Florida Keys [3] in a jam-packed day that has you eating breakfast in Key Largo [4], lunch in Marathon [5], and having dinner and drinks along Duval Street [6] to wind up the day.
Such an itinerary is ill-advised for all but the most time-pressed traveler; the whole point of making your way to the Keys is to experience the laid-back tropical vibe, and rushing through the islands is almost as bad as not seeing them at all.
If time is a factor, you should make a beeline directly for Key West for a weekend; two full days should allow you to take in the historic sights, do some snorkeling, and enjoy a couple of beautiful sunsets before hitting the bars along Duval Street. The Key Largo and Marathon areas are both worth a day or so each, but diving the reefs or heading out for some sportfishing require at least a day dedicated to each.
Links:
[1] http://www.moon.com/destinations/florida/miami
[2] http://www.moon.com/destinations/florida/florida-keys/key-west
[3] http://www.moon.com/destinations/florida/florida-keys
[4] http://www.moon.com/destinations/florida/florida-keys/upper-keys/key-largo
[5] http://www.moon.com/destinations/florida/florida-keys/lower-keys/marathon
[6] http://www.moon.com/destinations/florida/florida-keys/key-west/sights/duval-street