You will need:
a large supply of small seashells
colored sand
glue
firm tagboard of any color
This project can be completed in a center or whole class project. Each student will receive a small amount of sand in several colors. Colored sand can be put into salt shakers for easier use. Salt shakers work well in centers, but if you're doing this with the whole class, use small Dixie cups. You may also use natural sand, and just give the student a small plastic bowl or cup with sand. Students will also receive some small seashells and plain white glue, as well as a piece of hard tagboard in any color.
Students will squirt glue in a random pattern all around the tagboard. (This project requires quite a bit of glue.) Next, the students will just place their seashells on the tagboard wherever they desire. Finally, they will sprinkle sand over the remaining glue. The colors will mix and swirl together, forming a pretty design with the seashells. Just let the seashell art designs dry for a couple of hours and they are done. You may wish to spray the finished product with a varnish to seal the sand and make this project a little more durable.
This project can be a bit messy. Sand is a lot like glitter. It goes everywhere. If the sandy mess is a concern, I would suggest doing this as a center activity, and having the child sprinkle the sand over a small plastic tub to catch the sand. Also emphasize clean-up techniques that keep the sand contained. You could even opt to do this project outside at a picnic table if this is available.