I’ve used apple, pumpkin, and shapes skittles experiments for 18mos – 8 years old. I keep Skittles on hand for a last minute go-to.

https://www.playdoughtoplato.com/magic-pumpkin-science/

Variate on the pumpkin theme by making Squash monsters.

https://www.notjustahousewife.net/squash-monsters/