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 a fish? There's some great information here. https://overlordoftheuberferal.wordpress.com/tag/tessellation/

All the models are equipped with air holes that will help break the suction that when you're trying to demold. It has plagued sandcastle makers for generations. The holes are positioned so that you can place a finder over them during filling, although wet sand isn't likely to fall through anyway.