Photograph the same subject at three different times of day — compare the moods.
Light is the actual subject of every photograph, and the only way to learn it is to watch the same object change under it. Morning, noon, golden hour — three completely different photographs of the same thing. The exercise teaches you that the subject does not have to change for the picture to change.
Pick something stationary and unremarkable. A bench, a fence post, the side of a building. The more boring the subject, the more clearly the light becomes the story. Compare the three shots side by side and notice what each version emphasizes. That comparison is where the lesson lives.