Cross-Barred Venus Clam
You'll find this shells everywhere here in Florida. They're quite common. What I like about them is flipping them over to reveal a colorful underside. Colors range in hues of brilliant yellows, purples, and oranges.
I've been in love with having sand between my toes since I was a teenager taking the ferry across the bay to arrive at Presque Isle State Park (aka The Peninsula) in Erie, PA.