I 'scraplifted' this idea from MarthaStewart.com, so I will give those designers the credit.
It's a very simple idea made with envelopes.
However, here are a few more ideas I came up with:
If different sizes of envelopes aren't available, just use what is.
Papercrafting paper could also be folded into envelopes. Another idea would be to use white envelopes and have the young people color and draw on each one. Add numbers 1-24.