Driving through the Keys on US 1 is an amazing experience. If you're lucky enough to have good weather spend a little extra for a convertible. Driving over the bridges surrounded by clear blue water feeling the warm breezes was an experience I'll never forget. We spent the entire drive there and back saying "I'm so glad we decided on a convertible." It also comes in handy when you want to take some photos while driving!
Just to note, the drive takes longer than one would expect! Traffic along US-1 from Miami to Key Largo is pretty heavy. Until you get to right above Key Largo (the everglades), it is mile after mile after mile of strip malls and stop lights. Look into taking the highway or turnpike. I thought US-1 would be more scenic than it was. It doesn't get good until you enter the keys.
If you flew into Miami or Fort Lauderdale keep rush hour in mind! (We did not and missed our flight home due to a huge accident on 95 in Lauderdale)
And also remember... people in Miami drive like maniacs.