The only choices for hotel lodging in Key Biscayne are the Ritz-Carlton (completely unaffordable for the average joe) and the Silver Sands Resort (great off-season prices and moderate seasonal prices). I stayed there for a weekend orientation at my job and found it to be delightful. We stayed in the Cabins which are a little more expensive but more than worth the extra money.
The gardens are lush and beautiful, the beach is small but you can walk all the way to Crandon Park Beach if you want a bigger beach.
Crandon Beach has cabanas you can rent, a small botanical garden, an outdoor skating rink and lots of picnic areas. It is also within a few blocks of Bill Baggs State Park which has one of the nicest beaches in Miami and a tour-able lighthouse and light-keepers cabin. The pool is clean and large.
I was able to see some of the rooms that were by the pool and they are large and include a small refrigerator and microwave though they are definitely stuck in the 1950's with their terrazzo floors:)
If you are lucky, you might get to see hundreds of large iguana's sunning themselves out back!
The Island has several restaurants nearby and one of the nicest Winn Dixie's I've ever seen. If you do go to Bill Baggs, there is a restaurant at the No Name Harbor (yes, that actually IS it's name:) that has beautiful view of the sunset while the boats are coming into the harbor. There is also a small self service restaurant facing the water on the actual beach. They also rent some multi person bicycles to tour the parking area and you can also rent chairs and umbrellas right by the water.
Definitely stay in the Cabins or facing the courtyard.
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This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.