Curves

If you haven't used the "Curves" tool yet in Photoshop (Image-->Adjustments-->Curves), it's a great way to enhance your images with contrast, saturation and levels all in one tool.

Your assignment is to experiment with Curves by taking an existing image and creating 6 different variations of that image using nothing but the Curves tool.

Of the six images, one should be a "high key" image (high contrast, few shadows) and one should be a "low key" image (dark shadows dominate). 

http://chamee.deviantart.com/art/High-Key-Low-Key-104107453

I would first look for some examples of each--high key and low key--images on the web so you can recognize them while you experiment with Curves.

The other 4 variations can be whatever you find visually interesting.

Here is a tutorial on making a quick transformation to a high-key image:

https://williambeem.com/high-key-post-processing-in-two-easy-steps/

Here is a tutorial on making a painstakingly long and complex transformation (with fabulous results) to a low-key image.

https://medialoot.com/blog/how-to-create-a-dark-dramatic-photo-effect-in-photoshop/


There is a plethora of tutorials available on the Web for learning the intricacies of the Curves tool, but as in most things Photoshop, there is no substitute for experimenting.