Let's Tesselate!

These are some small sand molds that let you make tessellations on the beach.

The kite and dart were invented by Roger Penrose and let you fill a plane aperiodically. http://mathworld.wolfram.com/PenroseTiles.html

Rep-tiles are shapes that can be tiled in a way to make a larger version of themselves. This means they are also capable of filling a plane, as you can always make an exponentially larger Rep-tile from the one you just made. What Rep-tile is more appropriate for the beach than