Not sure how you would fit this in the box but I just love the idea of making a miniature garden (Ideas here).
This was something we always did when we visited my grandma. She would collect sand from the river out the back of the property and we would then put it into old margarine containers or foam meat trays, dampen it and then "plant" it with plants from the garden. They never lasted for long but we had lots of fun making it. These gardens look more longer lasting. Could lead to a unit study on Bonsai if the child was really interested in miniature gardens.
This would also make a good go along when reading Milly-Molly-Mandy as from memory in one of the books Milly-Molly-Mandy and Billy Blunt make a miniature garden to show at the show.