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.
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.