I take it you're on the AS flight to MIA? If so, depending on your interest in architecture and nature, I would recommend you head to South Beach to have dinner (Puerto Segua on 4th@Washington for Cuban is my personal recommendation) and take a look at the neon-lited art deco hotels and buildings as you'd be stuck in rush hour traffic headed out of Miami otherwise if you do arrive at 5pm. After that, just head back on the highway and head for Homestead/Florida City to stay at the Hampton Inn at the end of the turnpike (acceptable place as long as there's no stock car races then). Next morning, go to the Royal Palm visitor centre in the Everglades NP (highly recommended just after sunrise - nice cool quiet and lots of animals, birds and fishes to see before they're disturbed by loads of tourists) before heading downt to Key West. Scenically, the drive down the keys is disappointing for the most part (70% and up) and seems to be bordered by strip malls more and more these days (comparing drives in 1991-2 and 2000).

My sister stayed at the Sheraton earlier this year and thought it was o.k. (she is quite fussy). I might have stayed there years ago when it was a Sheraton (if it is the same hotel).