Your character is in a terrible mood. What could possibly cheer them up?
Write a scene that starts with your character experiencing a negative emotion. This can be anything from depressed to angry. You pick depending on what you already know about them. Something happens that cheers them up and switches the mood around completely. What is it? Who is involved?
Now you can do this the other way around and write a scene in which they are in a great mood and end it with them being back to black! What could happen to create such a switch in your person?
If this writing exercise is hard to do it might help to do other exercises first that help you to get to know your character better like The Proust Questionnaire or The Shopping List. They will clarify what is most important for your person or what their biggest nightmare would be.
In a classroom or course you could turn the choosing of the emotions into a game. You could for example print the list of positive and negative emotions and cut them out. Put them in two bowls, hats, buckets or whatever you can find and let each participant pick one of each without looking. Let them read the emotions they picked out loud. When everyone has their two emotions the group can start writing.
You could also have the group choose two emotions for their classmates. Chances are they will pick the hardest options and challenge each other. It can be easier to come up with a story when you are limited by the assignment. If there is less room, you have to get creative!