Pick three objects with nothing obvious in common — a shoe, a spoon, a toy car — and line them up.
Start a story using the first one and stop after two sentences: 'the shoe was walking down the road when suddenly...'
Hand over to your child. Take turns, each adding a couple of sentences, until all three objects have appeared. Say the whole story back at the end.
Building a story around fixed elements forces coherence: each turn has to connect to the last and still reach the remaining objects. That is narrative structure, the skill that later separates a child who can retell a book from one who lists its pictures.