Have you ever made candles from crayons?... Or a big soap from soap
A multi-colored oil pastel can be made from the stumps using a *very*
small quantity of mineral spirits with a oil paint drying agent added as a
solvent and binder.
Press the softened stubs into a mold (Anything that will not disentgrate
in the presence of mineral sprits, so some plastics are disasters waiting
to happen... glass is good.. I've used a butter dish before...) and leave
to set (press very firmly into the mold).
When the newly minted pastel is firm and dry, you can wrap the outside in
wax paper and duct or electrical tape and you've got big fun.
I would recommend sorting the colors by family so you don't make a brown
mess. Some really beautiful backgrounds can be made quickly with these