DIY Modge Podge - fill jar half way up with white glue, then fill jar up with water. This is a mixture glue/water. It will have a Matte finish. If you would like a Glossy finish, add 1 TBS of clear varnish. Note: may yellow over time.
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?