For the life of me, I cannot remember where I saw this. I promise, I'll look for the link. It is from a company that does clinics for teachers (public, homeschool, daycare teachers, etc) on different ways to do art with children. It basically teaches you to think outside the box and look at everything you see as a potential tool for painting, science, and experiments. With that said, I'll get on with the post.
For paint brushes, I used old crayola markers that had dried out. left the cap on and used duct tape to tape on things like strands of rubber bands, q-tips, and strands of a mop head. These were very quick and easy and a very green project. Though not in these pictures, I have done fly swatter painting in the past as well as painting with turkey basters.
I also used old nylon stockings (you can get these for like 33cents at walmart in little plastic eggs at the end of the aisle in the women's sock section.) I put popcorn kernels inside and tied it up. It becomes very "slinky" and leaves a great design. A funny note about this one is that we finished painting and I washed the stocking w/corn clean of paint and set it in a pot on the back patio because we were going to use it again soon. Sure enough, I forgot about it. A week or so ago, I found it and it had TONS growing in it. We were growing corn! How funny! The strange part is that it obviously wasn't being watered and we are in a drought. The stocking was too thick for the corn to be able to push through so it on kept growing and stretching the stocking. I wish I had taken a picture of it. The growths were all about 5 inches long at least and only about 10-15 of the kernels hadn't "bloomed."