As tides go out they often leave textured wave patterns embossed on the sand. Capture those shapes by photographing or sketching them. Copy the pattern onto thick paper. Divide blue craft paint into two containers and add water to one to thin it. Paint the wave pattern using a thick soft brush alternating between thick paint and thin paint for each wave. While the paint is still damp carefully scatter a thin layer of sand over the waves in thick paint. Leave to dry and blow away the sand to leave reveal the pattern it's created.
Collect empty containers of different shapes and sizes from home and take them to the beach. Gather shells, seaweed, driftwood and other washed-up items. Take damp sand, add extra sea-water if necessary, and fill the containers till the sand is compacted inside (as if making a sandcastle). Arrange molds in-line to create a circle, square or other geometric shape. Carefully turn over and lift off the molds. Arrange the other items to create abstract images inside the sand "frame."
Lay a piece of thick paper on a flat surface. Thin blue craft paint with water till it is very runny. Splodge the paint onto the paper and use a drinking straw to blow it across the paper starting from the middle of the splodge and working out in different directions. Repeat until the paper is covered in blow patterns. Leave to dry and cover paper with a layer of PVA glue. Take small handfulls of sand, hold them above the paper, and gently blow till the sand falls on the paper and sticks in different patterns. Leave to dry.
Use damp sand as a giant "canvas" for an art installation. Working close to the tideline, while the tide is out, use a stick to draw an image in the sand, which will be progressively erased as the tide comes in. A row of figures would make a good subject for this project. Start with the sketching the smallest figure nearest the water and increasing the figures' sizes as you work up the beach. Wait with a camera and photograph the image as each figure is washed away. Arrange developed photos in a row to show the installation's progress.