If your kids like to do craft painting projects, use an egg carton as a palate. Squirt your paints into the compartments and use the lid as a palate.
If you use the foam cartons you can rinse them out and reuse them.
If beading is their thing, sort all of the different beads into the compartments, wire or string into another and you've got the beading materials neat and organized and they can be stashed away easily when you're done.
Little embellishements for paper projects can be stored in the compartments too. Separate thread and sewing notions into the compartments a handy sewing box.
Mismatched bolts, washers and nuts can be used for craft projects too, stash them in the egg carton and you're all set when they need them.
All you have to do is label the end of each carton with its contents and stash them out of the way for easy access when the crafting bug hits.
Some crafty uses for egg cartons, I'm Mrs. Fixit and it's just that simple!