Sanibel Captiva Map. Sanibel island has 15 miles of beaches, 22 miles of bike paths, abundant wildlife and the largest undeveloped mangrove ecosystem in the country. So there’s more to Sanibel island than just shells! Well, not technically. The island is actually made of shells.
Cayo Costa State Park The best shelling beach in the U.S. is not Captiva or Sanibel, the twin islands just over the mainland from Ft. Myers. There's one much better, located two islands to the north of Sanibel as the seagull flies. Nine-mile-long Cayo Costa may lack a bridge and chic hotels, but this state-park isle makes up for it with a staggering array of hard-to-find seashells.