1) Keep a box with scraps of wrapping paper, pages from magazines, fabric, yarn, buttons,........to use for collage material. 2) Cereal boxes are great for making the covers of handmade books. 3) When you get a package in the mail, flatten the box and save the corrugated cardboard. It's a very sturdy, lightweight surface that remains stable for any number of projects. 4) Newspapers offer an interesting printing surface, although I realize many people read all their news online. 5) Kitchen sponges can be turned into ink pads. 6) Items with interesting shapes and textures can be turned into stamps for printmaking. Things like old keys, doilies, the bottom of an egg carton, leaves......can be printed.
Please feel free to leave any of your suggestions in the comments section. I've even re-used supplies left over from household re-modeling projects. The leftover grout from tiling one of our bathrooms was used to make a mosaic tray. I hate throwing stuff into a landfill.