My parents winter in Ft Myers and my father and stepmother live in Key West. Cant say much about FM without some parental input. As far as KW goes, it is fun just to walk around. One of my favorite stops is tne Peppers Store - lot of great sauces if you are into that sort of thing. Kermit's Key West Lime Shoppe is also excellent for candies, sauces, etc. If you are pressed for time, the Conch Train is a good way to see the tourist spots - goes right by my dad's house on Grinnell. Hemingway House is also a must.
My favorite eating spot is Louis' Backyard which backs up on the Atlantic. Michael's is a close second - now offers half portions. Turtle Kraals and Schooner Wharf are good lunch spots. I also like Finnegans for a little Irish fare.
Any B&B is probably fine - I only ever stayed in one, the Banyan, and enjoyed it. You might want to think about staying in old town but off the main drag as it can be quite loud late into the night.
Enjoy!