I would choose the activity the sun and the shadows as I think it is an interesting and practical lesson.
We would engage in the book mood bear and his shadow as a class. Explore and question how shadows get bigger and smaller and what causes this. We would then use the challenge cards and pose more challenge to the children. Outside we would in the morning use the position of the sun to draw a childs shadow and come back later in the day and redraw to show how the childs shadow has moved due to the position of the sun.