It's the perfect Mummy's-cooking-dinner activity! She sits at the bench and chats to me about the shells as I flit around our kitchen and shoot questions at her: what colour is that one? How does it feel (smooth/rough)? Is it plain or patterned? Can you hear the ocean in it?
If you don't have little drawers to hand, try containers, boxes, little bowls, platters or even glass jars with screw top lids for older toddlers - they'll love opening and closing the jars, but just make sure the jars are on the floor so they won't break if they're mishandled.