Sanibel is known for a few things, but the principal one is probably shells. If you haven't been there, it's hard to understand just how many shells there are. I would guess, with no exaggeration, that there are probably something like a hundred million shells of various sizes along a few miles of beach -- thousands of small ones in every square foot. Shelling is a standard daily activity, and jokes are rife about the chiropractic condition known as "the Sanibel Stoop".
Sandcastles are also common, of course, because of all the lovely soft sand, but the real fun is the occasional great bits of Transient Sand Art. For example, this detailed topographic map of the United States, complete with larger shells for the major mountain ranges:
Sanibel is also famous for sunsets. I suspect that they're less exciting for people on the left coast, but good sunsets are a rarity in my experience. Photos can't possibly capture them...
... but just to satisfy the desire to go snappita, snappita: