Here's a fun experiment using an old hot plate or warming tray, canvas paper, and a box of crayons or oil pastels.
If you don't have a warming tray, you can often find one at a thrift store for a few dollars. That's where I found mine. Plug it in, place the paper on top, and start to sketch with the pastels or crayons. You can even mix them. As the tray warms, the colors will begin to liquify. Oooh la la! Now the fun begins.
Your once solid crayon or pastel now becomes your paintbrush. You usually have to press hard to start, but after awhile, your touch should be light. If you find the colors are getting muddy or running together in a way you don't like, lift the paper off the tray and let the paper cool for a few seconds. The first time I tried this I didn't care for my results, but I kept at and now I love it.
Just a caution - I've never burned myself on the warming tray, but then again my husband says I have hands of asbestos. The old trays do not have heat adjusters, but ordinarily they don't get really hot--just warm. In saying this, I would not have younger children do this activity unsupervised.