My children have always loved this activity. Although we do have bought molds, the kids like to make their own using their favourite solid toys.
First get out the old batch of playdough from the fridge.
Roll it into a smooth ball and then shape it to a size just a little bigger than the toy.
Get your toy and squish!
and you have your mold.
Then pour in the plaster of paris.
Once dry, peel back the playdough and leave to dry further.
You can also use with natural 'toys' like pine cones, acorn cups, fossils, shells and even animal footprints. The soft playdough can be replaced with plasticine for a more solid mold although more expensive.