Key Biscayne, Florida
Right offshore, a vast sandbar makes for endless wading, and on the beach you'll also find a baseball diamond, volleyball courts, and a nature center with trails through the adjacent mangroves.
T&L Tip: Rent a private beach cabana on the park's south end (305/361-5421; $20 per day).
Check-In: The Ritz-Carlton Key Biscayne (455 Grand Bay Dr.; 305/365-4500; doubles from $269) has sandcastle competitions, sailing lessons, and seashell hunts.