Here on Nantucket at Christmas time many home owners make shell wreaths from scallop or mussel shells. This is done on a Styrofoam or cardboard ring mold. The shells are arranged and glued in concentric circles around the ring with shells overlapping one another. If you like this idea you might collect your shells now while you're on vacation and use them later in the year when you are home and crafting for Christmas. If you can draw simple designs, paint the inside of scallop shells for holiday tree ornaments.
Greeting cards are easy to make as well.
If you take a walk along the many paths leading to Cisco beach or out at the pine forest collect pine needles and tiny pinecones to decorate the front of a plain greeting card. Small shells can enhance one corner of a card as well. Take a picture of your family on vacation at a Nantucket beach and use this as your holiday card. Then decorate the card with a natural found object, perhaps a pressed flower from the island, and it will create a nice sentiment.