I save egg cartons of both kinds cardboard and Styrofoam. I use the bottom of all sizes in the Styrofoam to use when I do watercolor painting. They can be used for any painting medium and also for crafting. I use them and then toss them out after my day of painting for mixing colors.
You can also either the Styrofoam or cardboard cartons for all sorts of crafting or sewing, for storing beads for Jewelry making, for any kind of small items to keep you organized.