At the end of every school year my kids come home with a bunch of broken crayons in the bottom of their school supply boxes. Every year we try to find a use for them. I was stumped for ideas so this is all we did with them.
You just peel the paper off the crayons, sort the colors, and bake them at 350 for about 10 minutes in paper cups. If you want the to stay perfectly round you can use a muffin pan but a cookie sheet holds twice as many cups so I never do. You could also use candy molds in the microwave but they don't come out as nice.
The kids love watching them melt in the oven. When they come out of the oven the colors start swirling together and then separating again as they cool. It's sometimes very entertaining to mom too.
When they've cooled you have a bunch of crayons that look like rainbow peanut butter cups.