Carl,

IM gave one of the best routes to takes for fun and sight seeing / cool waterfront joints. It's a little time consuming, but it's a total S.Fla experience. Coconut Grove is kool also.
But it's alot to squeeze into a 'late am to 3pm in the Keys' ride. Maybe stay a night before hitting the S. Keys? From Ft. Laud Airport to the Key West is a few hours by car on a straight drive. How far south are you planning ongoing?
Not matter what you decide, enjoy the ride, and watch out for people in cars! Seriously, have the eyes in the sides and back of head alert while getting thru Ft. Laud and Miami, and you'll have a blast once you get into the Northern Keys. And it's absolutely gorgeous!
Bring the shades!