This simple beachy craft can be done right on the sand. At home, school or take a box of plaster of paris along to the beach. Form about a 3 X 3 inch square about 1 1/2 inches deep in damp sand. Use petroleum jelly to grease up the inside ridges of a scallop shell. Press the outside of it into the sand in the bottom of your square. If the sand has bits of shells and seaweed in it, that's fine. It'll make an interesting surface. Now fill the sand "box" with plaster of paris. Let it sit for an hour, then carefully remove the block of plaster of paris. Let it sit for several more hours to fully harden. Pull off the shell to see the cast underneath.
If you use a clam shell, grease the outside of it where it has ridges. Then put it in the sand hole with that side toward you (up). You can use a ruffled, rough oyster shell this way, also.
Scratching in the child's name and date makes a nice memento.