Grouping in the grass! (Or sand. Or amongst the trees.)
The Basic Idea
Get your class to understand the structure of multiplication and division by going outside and making collections of objects that are based on different calculations.
For example, show or tell them a calculation such as 12 x 3, or 16/4 and the children collect natural materials and make the groups that the calculation represents. This could be in a given time to add a bit of urgency to the proceedings.
How to take it even further or make it more challenging
Create arrays and attempt some large scale grid calculations.