When I was little I had a set a set you could use to take casts of animal footprints- it had plaster of paris and a sort of plastic ring that you placed over the print to hold the plaster in. Never worked very well because it relied on the ground having been wet enough for the print to be formed but then dry enough for the plaster to set but anyway, I loved it.
My niece would love something like this but I can't see anything similar for sale. Has anyone seen one?