The saying “throw sand in people’s eyes” means deception. However, the sandman also throws sand in people’s eyes so that they can get a restful night’s sleep. In addition, sand also points to the element of earth (in a more delicate form), and in that sense again points to the dreamer being grounded.
In the form of sand on the beach, this image points to vacation and relaxation and, in addition, the gentle waves of emotions that over time smooth out that which is rough and tough (stone).
If this dream takes place in a sandbox, your unconscious wants to give you a plan that you can use in your everyday life.