I saw this idea a while ago and have been meaning to put it into practice. It worked a treat and it was the perfect half an hour activity for after school.
You need to snap crayons into small pieces and then place them into an ice cube tray. I asked Ana to combine the colours as she pleased and that single task took her a good 15 minutes. Then we popped them in the oven for about 10 minutes where they melted. Let them cool for 30 minutes or so and voila... recycled crayons in multiple colours.