Beach blanket?

SAND

Think of what you might shake out of your towel after a day by the shore.

This clue plays on the double meaning of "blanket." While a beach blanket is something you spread out to sit on, the word "blanket" can also mean a layer that covers something completely—like how sand covers a beach. The question mark signals it’s a pun, hinting you should think beyond the literal object. So instead of fabric, the answer is the natural "covering" you’d find there: sand.