Magnetic Slime--really neat, but it requires liquid starch, iron oxide powder, and really strong neodymium magnets. There is a commercially available magnet putty that might be a good substitute for exploration.
These tin can robot kits reminiscent of Mr.Potato allow kids to create (and re-create) robots from recycled parts and magnets. All the "body" parts can be stashed inside the tin can, wrapped with a how-to labelâ€”perfect for a DIY party craft or gift.
DIY: Make your own magnetic play set for preschool