While the center of the island is a lively society of self-reliant residents, folks from all over Miami love to visit for the manner of exercise you can do here, especially golf and tennis at world-famous par-72 Crandon Golf Course (formerly the Links at Key Biscayne) and Crandon Tennis Center. The latter is where you can watch, every March, the Sony Open, one of the most important tournaments in the world outside of the Grand Slams. Another eleven courts, designed by Cliff Drysdale, are located nearby at the The Ritz-Carlton Key Biscayne, which also has one of the most enticing brunches in the city.
In addition to golf and tennis, enthusiasts adore Key Biscayne for the ocean kayaking, windsurfing and paddle-boarding opportunities. The bay is ultra-calm and shallow here, with many ports of entry and a multitude of vendors for rentals and lessons on both sides, and it's a joy to glide along so much wildlife: dolphins, manatees, bonefish, permit and rays, in particular. Or head to Miami Seaquarium for such close encounters, where unless you swim with the dolphins, you don't have to actually get wet.
For more nature, head to Marjory Stoneman Douglas Nature Center at the northern tip of Crandon Park Beach. The Park also is famous for its Gardens, which are diverse and deliberately maintained as both tame and wild, and its two-mile stretch of sugary white sand framing a gentle lagoon. For some manufactured fun, the Crandon Park Beach Amusement Center features an outdoor skating rink—more exercise!—and a historic, restored carousel, in addition to a playground and splash fountain.