I'm looking for a really nice place to stay for several days on Marathon Key - suggestions?

Banana Bay Resort.

Banana Bay is nice but maybe not REALLY nice. But it is really the only thing there. I would suggest maybe renting something.

Hawk's Cay and Little Palm Island (very nice and exclusive) are very nice properties.

Might want to check out Faro Blanco. They have a variety of accommodations.

Hawk's Cay is actually on Duck Key. About MM66 or so. In the area, certainly, but I think Marathon starts around MM55 and goes down from there. It'd be my choice for the area, if I wanted to spend the bucks and didn't need to be in Marathon itself. We stayed there ten years ago, and my wife still talks about it....

They are not on Vaca (Marathon) Key. I think they are 20 miles away.

Loved staying at Cocoplum Beach & Tennis Club (condos).
http://www.cocoplum.com/