Making paperweights or just something fun can be a great way to get kids involved in exploring nature, being interested in the natural world around them, and make something they can keep. The casting pictured was from projects I did with a three-year-old I took care of one summer (I spent everyday up to 8 or more hours with her, even traveling on vacation with the family as their nanny). We collected flowers, leaves, twigs, pine cones, and other bits from nature and mixed them with seashells as a fun way to learn about nature and fossils. (We did learning projects everyday that summer.) They could be painted when dry to add another element.
We used Plaster of Paris at one of my camps to make imprints of animal tracks. Mainly we found deer and raccoon tracks near places where they went to drink.