This is a great idea for older children.
Give each child two pieces of thick square card, one piece of card should have a square window cut out of it to look like a frame. Give the children paint, glitter, sand, crepe paper, chalks and sea animal stamps or stickers to decorate their card. Paint and decorate the frame with small pebbles, sand or shells etc and leave to dry.
Next get a small seal-able food bag and put a small amount of water in the bag, seal the top and fold over selotaping it to the top of the decorated card. Selotape the edges of the bag down to cover the card and glue the frame on the top to hide the edges of the bag.