We have stayed in this Hampton Inn which is right on the bay in Key Largo, all tile rooms, pool, great free breakfast, nicest Hampton Inn I have ever seen. It's in Key Largo tucked in on the right side headed south. It's just past the Fish House restaurant which is on the left if you are southbound.
The Hilton in Key Largo is another couple miles south also on the right is one of the nicest places we have ever stayed. Breakfast looking out over Florida bay is one not soon forgotten. Snook's Bayside another very nice place for a happy hour or dinner in Key Largo.
In Marathon a little more low key is The Black Fin Resort and marina. Not as plush, but clean, nice little pool, cute bar next door ,great laundry spot. Nice barbecues with picnic tables tucked in a number of places, nearby great breakfast at the Stuffed Pig at 36th street. Great dinners also on 36th street ay Key Fisheries at the end of 36th street at the end bayside. Get your food and go next door up on the second floor for nice views over looking Florida bay.
Finally in Key West we like the Double Tree Resort. Nice property great hotel, shuttles to down town Key West so you don't have to deal with parking, and worrying about having a few cocktails

I would try making reservations at a restaurant called 7 Fish, and Turtle Krall's near the schooner wharf area on Key West Harbor. Take a sunset cruise on one of the sailboats. Also high speed catamarans that run out to Fort Jefferson. Take the Florida turnpike from Miami to get to the Homestead area to get back on Rt.1. A little more traffic these days but still a beautiful ride. Have Fun.
D.D.