Super Mega Stencil Kit (from Oriental Trading)
I keep a stash of manila paper on hand, all the sheets of which are cut in half to make the supply last longer. Aside from that far-in-advance preparation, there is nothing else needed.
This set of stencils includes a variety of themes. For our first time using them, I just put out the fantasy set, and advertised it as an opportunity to draw a fantasy scene. The stencils are somewhat difficult to use, but the older kids had no problems at all, and the younger ones just colored without regard for the stencils. Some of the kids also used them as rubbings instead of stencils.
Next time: Cat Magic Color Scratch