Grassy Key is just South (west technically) of Duck Key and starts the beautiful city of Marathon. Grassy Key offers the Dolphin Research Center which is a 90,000 square foot series of salt water lagoons that have bottlenose dolphins and also California Sea Lions in which you are able to interact (different programs available to purchase) with the marine life.
There are a few restaurants in Grassy Key as well from The Wreck, Grassy Key Outpost and The Hideaway Cafe, along with a few motels right on the open water. There is a few residential areas in Grassy Key and vacation rentals.
Grassy Key you are able to rent for a 7 day minimum (vacation rental) with the proper licenses through the City of Marathon.