Show and tell is an age old concept, that in early elementary helps to teach children to talk to a group and take turns. Unfortunately it is often dominated by outrageous plastic toys. A few years ago I ran into a teacher who had the requirement that the object needed to be from the outdoor environment.
This created an endless array of sticks, rocks, bugs (in jars), frogs, snow and many other objects. The children still tried to outdo each other but no one seemed to tire of endless array of rocks, sticks, and feathers and their vocabulary grew along with their observation skills.
It might be fun to give it a try and see what fun items are brought to you.
This site has 30 ways to teach rocks to pre-school children:
Teach about rocks PreK-K