Biscayne is a magnet for diving, snorkeling, canoeing, birding, and, to some extent (if you have a private boat), camping. Elliott Key is the best place to hike.
Biscayne's corals range from soft, flagellant fans, plumes, and whips found chiefly in shallow patch reefs to the hard brain corals, elkhorn, and staghorn forms that can withstand depths and heavier shoreline wave action.
To the east, about 8 miles off the coast, 44 tiny keys stretch 18 nautical miles north to south, and are reached only by boat. No mainland commercial transportation operates to the islands, and only a handful are accessible: Elliott, Boca Chita, Adams, and Sands keys, lying between Elliott and Boca Chita. The rest are wildlife refuges or have rocky shores or waters too shallow for boats. December through April, when the mosquito population is less aggressive, is the best time to explore. Bring repellent, sunscreen, and water.