Put down your racket and cross-train at the oceanfront fitness center, which offers everything from a two-mile guided run around the island to traditional kickboxing sessions ($15 per class). Next, hit the water. Kayaks, windsurfers, and sailboats are available at the hotel ($25 to $95 an hour), or rent a kayak at Biscayne National Park and paddle through the mangroves ($16 an hour; nps.gov/bisc), where you can spot more than 170 species of birds, like blue herons and snowy egrets.
DETAILS Rates for the Island Tennis Getaway package start at $499 a night per person and include accommodation, valet parking, breakfast for two, and unlimited court time. Go to ritzcarlton.com/resorts/key_biscayne, or call 800-241-3333 for more info.