South - In Islamorada - Robbies to feed the tarpon fish. Lunch or dinner at Lorelei's has a great view of Florida Bay.
Stopping and seeing (not feeding!) the key deer is cool.
https://www.fws.gov/refuge/National_.../key_deer.html
I would go south first, as Fantasyfest weekend in Key West is that last weekend in October and it is crazy and rooms are expensive and hard to come by.
I would take Card Sound route into the keys. View from the bridge is spectacular.
North - Take A1A north. Lunch at Aruba's in Lauderdale by the Sea has a spectacular view.
Will add more later.