Arrange 8–12 plastic cups or yogurt containers in a circle on the floor, open in the middle. Put a few toy animals nearby.
Place one animal in the middle. "It's in the pond."
Kids add animals to the pond. Move cups closer or farther. Knock some over and rebuild. A 2-year-old will mostly tip cups; a 4-year-old will narrate a whole ecosystem.
Use the cups upside-down as "islands." Two cup rings = two ponds.