On a day near the end of the school year, when I found myself substituting in a second grade classroom where I had never been before, for a teacher I had never met, in an emergency situation in which there were no lesson plans, the first thing I did was scout around the cupboards to see what art materials might be available. I found some white construction paper that had been cut to about 9x18, so I pulled it out, sorted through it for pieces that were not bent or torn, counted out enough sheets for the class, consulted my little folder stash of emergency ideas, and decided to have the students do some line designs with their hands.
I demonstrated on the white board what to do: I traced my hand along with part of my arm, then drew five lines across the paper in different directions, crossing over the traced hand and extending the lines off the edges of the paper. I gave directions to color in all the spaces however they liked, and let them go to work. It was interesting to see that some students drew all their lines extending off two edges of the paper, while other students drew some of their lines stopping at other lines rather than crossing them.
I could see doing this using only warm or cool colors, or warm colors in the hand spaces and cool colors in the background spaces, or patterns in the hand spaces with plain colors in the background spaces (or vice-verse), or using only three analogous colors, or different values of one color, or using watercolors, or pastels, or adding textured rubbings, or doing it with two hands, or three hands....
I'm pretty sure this is something I will do again. It was easy, fun, interesting, and needed no planning whatsoever!