I have a bunch of strong magnets from old hard drives and I am thinking hard how this could finally be a good use of them. But the tube is opaque, so I can't see how I can make a toy out of it.
Put strong magnet in small tupperware box. Put box on beach, collect fines of magnetic material. Transfer many fines to a clear pot, mix with mineral oil. Tape strong magnet to outside of clear pot, see magnetic field in 3D.
Magnetic visualizer sprays are also great fun -- assuming that you can still buy any of them. You can actually see the magnetically recorded data on floppies, the magnetic stripe on ATM cards, hotel card keys, driver's licenses, and analog data on audio tapes. In the case of digital data, it will look like bars and blanks. If you know the encoding format, you can visually figure out what's encoded there.
Some commercial products were Magview, Magcheck, Ferro-see, and Sprague-Mag. I don't know if any of these are still available.
The spray is iron powder suspended in a fluid (like trichlorotrifluoroethane) that evaporates rapidly and leaves magnetic particles oriented according to the magnetic field. The pattern can be made permanent by spraying with a fixative or hairspray, or lifted with cellophane tape.
