Paint a large cardboard box with acrylic paint using a large brush. Give it stripes or stars or any other brightly colored decorations.
Throw old clothes into the prop box. While large shirts and pants are a good idea, try to include clothing that is representative of a profession, like a white lab coat for a scientist or an apron for a chef.
Add a bundle of silk flowers into the box. Flowers may be used in skits or as decorations for the costumes.
Secure artificial money together in a bundle with a rubber band. The money can be traded back and forth between the children, teaching them more about math and commerce.
Add several yards of different types of fabric to the prop box. The fabric can be used to cover tables or it can be used to create togas and other types of costumes.
Place paper items like menus, maps and awards certificates into a folder and place the folder in the box. These props help set a sense of place.