We did some blocks as a class exercise a while back. It was stressed to only use Crayola crayons - apparently something to do with the wax content of them.
The FUN part was that we were all told to bring in items with 'rigid texture' like lace or netting or a piece of the bumpy stuff you can use for drawer liners - anything you can imagine. Then, when it was time for coloring, we put the textured item under the spot we wanted to color and colored over it. It made wonderful effects on the muslin. For instance, instead of just coloring a sun hat - we put screen under it to give it a straw-hat look, etc. It was a lot of fun, and easy to do, and you could go on to embellish it with embroidery or marking pens or beads. The possibilities are endless.