Edit 11/05: I added a picture of my watercolor leaf sketch to be used as an example/reference.
I sat outside with my art journal and sketched a few leaves that I found in our yard. I thought this would make a lovely sketching prompt for the next few days or week.
If you haven’t sketched a leaf before, look around outside and find one or two leaves that have a simple shape. Identify the basic shape(s) of the leaf and sketch the outline. Next, sketch in the veins of the leaf. Don’t forget to add any cast shadows you see under the leaf.
Here’s a quick watercolor sketch of my leaf.

Besides the typical fall colors you see in the leaves try using a unique inky color or even use your favorite ink colors. How about a teal leaf? Or a pinky rose leaf? Or a multi-color leaf?
Challenge: to help get into the daily sketching mindset, find a different leaf each day and sketch it.
Have fun!