I used to make magnets with my kids in class by getting various moulds and a plaster of paris mix (can get from Crazy Clarks or any art shop)
They would measure out the water and powder and mix it and then they would pour it into the moulds. It was also a great thing to do when teaching solids & liquids and how liquids can become solid
Once set I took the things out of the moulds and let them decorate them with paint & glitter and then cut a strip off magnetic tape and stuck it on the back.
We made lovely Xmas fridge magnets as pressies for parents this way.
NB: instead of magnets you can also stick a paper clip in the wet mixture so when it solidifies the paper clip sticks out of the top and you can make Xmas tree decorations instead
