5 gallon tank full of active live sand (as many worms, snails copepods etc that you can manage) and some hermits, maybe some algae, etc. maybe a piece or two of LR
Demonstrate the ecosystem of a marine reef.
Talk about the food chain from the fish at the top to the bacteria at the bottom, etc. Show how the reefs are self sustaining and self maintaining.
Then discuss how delicate the balance is. Removing any one part of the pyramid and the system falls out of balance.
good and "sciencey" and will also get you environmental awareness brownie points