Have the kids think about pencils, crayons, markers and pens. Encourage kids to plan an invention that will use these things in a different way or make homemade rather than purchased writing utensils. Kids can also think of ways to keep the round things from rolling off a slanted desk, fit into a pocket, fasten to a hat, or clip to a keyring.
For younger children, provide scraps of craft foam or Styrofoam and have them think of an invention to keep pencils from rolling off their desks. Older kids can take apart a ballpoint pen and find different uses for the empty pen, such as a spyglass or spy camera.