Story Prompt: Show, Don't Tell
Okay, this isn't so much a story prompt as it is a scene prompt and practice for a general writing tip.
A common adage among writers is "show, don't tell". In other words, don't explain everything that's going on and everything character is thinking or feeling. Instead, use actions or perception to reveal what's going on with a character, internally or externally, and let the readers draw conclusions. For instance, instead of saying "Yoniko was disappointed," say "Yoniko's smile faded as her shoulders dropped."
Writing in this way prompts students to think of different ways to express the same thing. Students will have to use expressions and imagery - like native speakers do in most situations - instead of being straightforward. It may also expand their vocabulary usage (they've been using emotion words like 'happy' and 'sad' from a beginner level; let's see them use other words that they should know individually in the same context to provide the same meaning with a richer reader experience). Finally, it can help students with their reasoning skills.
Set the Stage
First, give the students something to write about. It shouldn't be a full story. More advanced students might want to do a short scene, but otherwise just a moment in time will suffice. Give the students a few things to work with:
a character - this could be a well-known character from pop-culture, or it could be original. The character could be well-defined or simple (eg. a police officer, an athletic woman, a boy who doesn't fit in, etc.)
a location, and maybe a time - what's going on around the character? Who and what can she/he interact with?
a focus - what is the character thinking about: work, a crush, an embarrassing situation, food? Maybe you prompt your students what a recent incident, and they have to tell you the character's response to it. It's up to you whether you dictate the emotion or let the students decide for themselves.
Now the students have something to write about. Again, there's no story progression (unless you want there to be), so what the students write might be pretty short. A single paragraph may be enough.
Here are some things for your students to consider in order to indirectly (yet adequately) display how your character thinks and feels.
Perception Indicates Thought*
What the character is looking at (or smelling, tasting, hearing, or touching) should tell the reader what the character is thinking about. If the writer tells us that the character can't stop looking at his pretty classmate, we know he fancies her. If the character is massaging her shoulder, maybe she's thinking about a recent injury. If he keeps looking at his watch, he's more concerned with the future than the present. If she's smelling food, she's probably hungry.
Breath Indicates Emotion*
How a character breathes reflects their immediate emotion. A gasp indicates surprise. A groan indicates annoyance. A sigh indicates boredom, annoyance, or relief. A gulp indicates dread. A chuckle indicates amusement. Held breath indicates fear or anticipation.
Body Language Indicates Emotion
Body language - including facial expressions - reveals emotions. Confusion may be conveyed by scratching one's head; disbelief by raising an eyebrow; suspicion by narrowing one's eyes; eagerness by leaning forward; reservation by crossing one's arms; etc.
Movement Indicates Intent*
How the characters moves, and what they move toward or from, informs the readers of the character's intention. If she moves toward another person, she intends to interact with him. If he reaches out a hand, he probably wants to build up a relationship. If she keeps shifting her weight, she might be looking for an excuse to leave. If he stops walking, he's probably hesitant.
I've given a bunch of examples of these four tools, but there are plenty of others I haven't covered. Before your student begin writing the actual scene, have a brainstorming session with them. Body language alone could take half a lesson; there are so many options.
Share the Snapshots
Once every student has written their moment in time - or their snapshot of their character's life - have each of them read it aloud. If you've given everyone the same prompts, it'll be interesting to compare how different students showed the same thoughts and feelings in different ways.
Another option is to give them all separate prompts. Then after each students reads their snapshot aloud, the other students have to guess what the character was thinking about, and how they felt about it.
* Some of the ideas for how to show thought, emotion, and intent come from the advice of Mary Robinette Kowal, a writer and puppeteer.