A couple of weeks ago we went on a 32mile (52km) round trip from Miami Beach to Key Biscayne. This whole area is ideal for cycling due to its flatness and reminds me of the North Germany or Holland: no hills, only the occasional bridge to climb.
If you go by boat it’s only 5 miles tops, but due to the man-made island setup we have here you can’t always go in one straight line.
The way to Key Biscayne has a lot of sights to offer and only mild to medium traffic along the way. Incidentally this is where “Tennis from Miami” comes from: Crandon Park is located there, and at the very south tip at Mile Marker Zero is Bill Bagg’s Cape Florida State Park. It’s so quiet there it almost hurts your ears!