Autumn is starting here where I live in Canada. It's the perfect time to go outside with a class and gather some leaves to draw. My Grade 4-6 students practiced observational drawing with their leaves.
I turn off all the lights in my room and just use the lighting from the window. You can also light with a lamp if you have one. Have them focus on creating a contour drawing first, then start shading.
The materials we use are drawing pencils, blending stumps and kneaded erasers (I only use the Faber Castell brand!).
This student sprinkled water on hers as she wanted dew drops.