I am thinking about ordering a sea shell garland that I saw on orientaltrading.com. It is made with raffia and has shells and starfish dangling down. I'm thinking about adding it to a valance. My mom has a huge glass bowl (sortof globe shaped) filled with shells. If you could find something less breakable to put a bulk order of shells in, that would be great for her to dig through. My DS would love that. Our dentist has a huge bowl of different kinds of rock and the kids always dig through it.
I know these are kindof random, but my DS made a craft in preschool that we have loved. It was just a water bottle filled with sandbox sand, small sea shells, pennies, beads, and other small treasures you might find at the beach. The cap was hot glued and then screwed on so it would not make a mess later. The kids just tip the bottle around to uncover the treasures inside. Sortof a "beach in a bottle" idea. Good luck!