Free
As a design teacher, I always keep an eye out for free building materials that kids could use.
On top of the list was cardboard and boxboard. Boxboard came from cereal boxes. I also collected the cereal box liners.
Wrapping and craft tissue paper, especially sparkly glitter tissue paper or sequin sparkle tissue paper. These were highly sought after for decorative purposes.
Free building material meant that I could give a design and construction assignment and my student had all the building materials they would need without cost.