In addition to lots of science learning in the sandbox, think of all the kindergarten readiness math. One understanding that children develop is that of more and less. 
While this seems pretty obvious to us, it isn’t to kids. They need lots of play time putting sand in a container and adding more. Or seeing that one container is full while another has less.
Older kidlets can be challenged by filling one container and then pouring into a different one to see if it holds more or less. Play activities can be extended by asking a child to see if 2 containers are equal and hold just the same amount. Which one holds the very most?
All of this playing and discovering builds important readiness for kindergarten. After all, math is a system based on more and less and equal. Do you agree there’s pails of fun and learning?