Watercolour triangles let students paint paper with watercolors, wet on wet, similar colors. Let dry and cut into triangles. Students pick triangles and glue down randomly, leaving white around each one. Maybe try with black or other color background?
Great project for first day of class. Give each student a pice of gridded paper. Color in one of the squares with a design then pass on to the next person to fill in the next square. Use Crayola watercolor pencils or markers on vellum.