This is just a very small thing that I figured out about Foundation and figured I would share. In hindsight it is kinda obvious, but I don’t remember it being mentioned anywhere in the Foundation docs.
When setting up columns in Foundation, you have three sizes to pick from: small, medium, and large. The trick, if you want the divs to stack on smaller displays is to set the size for the screen width were you want the columns to appear. So, if you want your divs to be stacked on small screens and columns on medium and larger screens, then use medium.