I actually stole this idea from the art teacher at my mom's school when I was studying to be an art teacher (amazingly talented art teacher), which I believe she got it from The Incredible Art Department. We painted all sorts of different colored paper and then cut those up to make these beautiful collages based off of Eric Carle's Very Hungry Caterpillar. The kids loved every step of this, my only struggle was getting them to cut the details out rather than draw them with pencils, so I started to precut some things to give them ideas, and that helped.
*Last year I did this with my third graders, and they did an awesome job in about 3-45 minute sessions.