The girls' interest in magnetism has primarily served as my excuse to spend all our homeschooling budget on all kinds of awesome magnets--teeny-tiny magnets, square magnets, spherical magnets, magnets that form a perfect circle when you link them just right, magnets so powerful that they have to be stored separately and played with carefully.
Along with the magnets, I've discovered a whole world of magnet accessories, not the least of which is this incredibly awesome material known as ferrofluid. Ferrofluid consists of nano-sized ferromagnetic particles suspended in a fluid and coated in surfactant--it's a magnet-loving fluid that flows.
I put a healthy dollop of ferrofluid into a Mason jar, filled the rest of the jar with a water/glycerin combo, and then glued the lid on (I've heard that ferrofluid can be QUITE messy, of the leaping out of its container and splashing all over the wall because you've got a magnet stuck there kind of messy).
To play, you run a magnet along the outside of the Mason jar, and the ferrofluid will be attracted to it, the power and shape and location and number of the magnets all adding new variables to explore:
Of course, there are a million more ways to play with ferrofluid--some of them involving open containers, gasp! And no, I'm STILL not done buying magnets!