We did a craft in Brownies once. I'm remembering this from 20 years ago as one of the kids doing it, so I hope I'm not forgetting anything crucial.
Take a sheet of wax paper. Gather some pretty leaves or something, maybe even cut nice shapesout of construction paper. Lay them on the wax paper, not filling it up, just a bit here and there. Grate crayon bits and sprinkle them on the paper however you want (ie, you grate them up and let the kiddos sprinkle). Lay another sheet of wax paper on top and you pass a hot iron overtop, not touching or you'll have a waxy iron, until it all melts together. It's stained glass!
I think we put some construction paper around for a frame, and put a string on it to hang in the window.
I know it doesn't use a lot of crayon up with only a few kids, but maybe you can do it as a group activity for an organization if your kids are in something (sunday school, scouts, 4H, etc)?