The Florida Keys is a set of tropical islands connected by a ~120 mi highway. At first, it may seem that it will take ~2 h to drive through the Florida Keys, but because of their low-speed limits, it takes ~3-3.5 h to go one way. Our first stop was Marathon beach (free with a lot of parking). Then we drove to the most touristic area - Key West. We wanted to see John Pennekamp Coral Reef State Park, but unfortunately, we couldn't spare more time. We wanted to take glass bottom boat tour there (2.5 h $25).
Tip: make a research about "Sunpass" Florida's toll system and potentially you can save some money and your time if you buy a portable receiver.
The main attraction of Key West is the Southernmost Point. People line up in a very long queue to take a picture there. If you don't mind walking you can easily find free parking in 0.5 mi from the point. From there you can walk to the Mallory Square and get some fresh coconut milk on your way.
On your way you will see an old broken bridge and it will make you wonder what its history is. Here are some details. And if you have more time and are not afraid of the heat you can ride a bicycle there.
After exploring Key West, we had to drive all that distance back to get to Miami. I think that traffic around Miami is one of the worst, lots of speeding and inadequate lane changing on their highways.