Sanibel Island:

A recent New York Times article writes "A family learns that Sanibel in Florida is habit-forming...Now with strict zoning and environmental regulation, the winter population rarely rises above 30,000. Nature preserves make up most of the island's acreage, the beaches are not crowded, and tourists like us can enjoy the best of both worlds...We rent bicycles and tour parts of the islands 23 miles of hill-free bike trails, swim in the pool, and comb the beach to our heart's content."