I enjoy so much making collages of all kinds with my students and specially when I find the best collage materials that cost me little or nothing. Here are some material ideas:
punched circles from a three hole punch
paper scraps from previous projects
tissue paper, ribbon & old wrapping paper recycled from gifts
old catalogs or magazines
drinking straw pieces
rocks, sticks, leaves, pinecones
dried grasses & flowers
lentils, small beans
all types of pastas
dryer lint (no kidding)
...and, of course, the obvious: glue, markers, crayons, construction paper and paints.
Encourage them to tear pictures or shapes for their creations. The imagination can definitely be sparked with the interesting shapes created by tearing paper. There's also the wonderful tactile experience and, of course, the SOUND.