The below are informal prompts used successfully in adult writing groups consisting of 3 - 10 people.

We used tried-and-true activities as well as made up some of our own. We typically spent five to twenty-five minutes per activity, then read aloud afterward.

• The 100-Word Story This is a popular writing exercise (and contest!) Squeeze enough elements to tell a story in a mere 100 words. Sometimes you are given parameters (like "both romance and betrayal are required") or given a title (real or made-up like "A Tale of Two Cities"). Search for "100 Word Story" on the web to find more examples.
• Thirty Words Along the lines of the above, write up to thirty words (or 3 ten-word sentences) stuffing as much information about a character, a setting, and their situation in as possible. Afterward, write the most vague, abstract thirty words you can think of.
• Word Play: Write Using Given Words Either come prepared with five to ten words (for example: fresh strawberries, an aardvark, Chicago, unrequited love, a postal worker, ten talking turtles, on a space station) or gather some from each participant on the spot. The words can be characters, places, objects, concepts, anything. Then write something using all of these words. Everyone can write using the same words, or choose words from the center.
• Word Play: A New Word Every Two Minutes A variation on the above exercise begins with just one or two words. Then, add a new word every minute or so, the writer incorporating the new concept in the story. (Bonus: choose a genre or feeling to write in.)
• Horoscope Stories Pick prepared horoscope clippings out of a bag and write about what you read. Add a character or setting from another part of the newspaper.
• Written Gesture Given a concept (for example: freedom, industry, eroticism, happiness), write a sentence or two that encapsulates this feeling for you. Description, a dialogue, a short scene, whatever works for you.
• Sense of Place Show a map (of Europe, Asia, etc) and have everyone imagine a place on it that they are drawn to, whether real or fictional. Write a story about that place, focusing on the setting.
• Metaphor Describe a room - any shape, any size, and with anything in it - using only metaphors, similes and comparisons.
• The Beginning Given the first sentence of a story, everyone continue it how they please. (For example: "Normally, people did not go to the cemetery alone after dark, however..." or "I've always heard the phrase 'if walls could talk' but I never thought one actually would.")
• The Beginning and The End Given the first sentence of a story and the final sentence of a story, begin writing. (For example, a story beginning with "Frustrated and angry, he caught sight of the dirty dishes in the sink and begrudgingly began washing." and ending with "When the last dish was clean, his mind was clear was well. The dishes had helped him realize what he needed to do next.") You do not necessarily have to end up at the last sentence, but having the ending in your mind as you write will help direct the story.
• Emotional Dialogues Given two or three characters (in our case, two people and a goldfish) write four short (unrelated) dialogues. For example: one dialogue is regretful, one is anxious, one is joyful, and one is erotic.
• The Moral of the Story is... First, imagine an animal in a place where there is an impending sense of doom. Next have another animal approach. Have the first one say something, the other respond, then the first reply again. End it with "The moral of the story is..." This activity is more interesting if you don't tell everyone that there is supposed to be a moral at the end.
• Visuals and Images Spread out a variety of pictures, paintings, magazine clippings, any interesting or bizarre image you can find. Then focus on one picture in particular and write about it. (Variation: pick two very different pictures and combine them.)
• Murder Diary There has been a murder, but it has been solved. Write the process in a series of short diary entries from someone involved directly or indirectly.
• An Explanation Given an unlikely statement or situation, write an explanation of how it can be true. (For example: we had two very different pictures of women. "These two woman are the same person. Explain.")
• Natural Disaster Write about a personal experience with a natural disaster, major weather occurrence, dangerous phenomenon, or the like.
• Stream of Thought Write for twenty or so minutes, no stopping, no looking back, no re-reading, no correcting, on a single simple concept (for example: summer). Not necessary to read the entire stream aloud, only parts you are proud of.
• A Problem to Solve First, write about a character who has a problem they cannot solve (it can be something minor like not being able to get the garage-door opener to work, or it can be more substantial). Then, have the character solve, or begin to solve, the problem with the help of an inanimate object.
• Book Covers Put some novels of a variety of genres in front of the group. Then, write a synopsis of the book based solely on the cover and title.
• DVD Covers Similar to the above, write the synopsis of a movie based off the cover of a DVD you have never seen. After that, write the "opposite" of that story, whatever that means to you.
• What Would Your Car Say? Just as the name suggests, write an argument, discussion, or hypothetical dialogue between you and your car.
• The Nature of a Word Given a word, write five sentences using that word in as many different ways as you can. Whether parts of speech, alternate meanings, etc, stretch the word as far as it will go.
• Opening Sentence First, read the opening sentence of a variety of books. (The Fellowship of the Ring, for example, has a great opening line) Note that the first sentence should make the reader ask a question. Then write eight to ten story openings (quickly, don't think too much) in about ten minutes. Afterward, have the other members of the group choose an opening for you to continue.
• Character Sheet Given prepared character sheets (which included name, gender, age, race, special talents, childhood trauma, greatest fear, greatest desire, and the lengths one would go to achieve that desire), have everyone fill three out. First, pick one and pick a setting (either prepared or not). Then write. Finally, take the other two and write a scene between them.
• Character Opposites Take some time to describe a character, any character. Then, after you finish, write that character's opposite.
• A New Fairy Tale Genre Pick a common fairy tale out of an envelope (red riding hood, hansel and gretel, cinderella, snow white) then pick a genre out of an envelope (detective noir, science fiction, slapstick comedy, shakespearean tragedy). Rewrite the fairy tale in the new genre.
• Fairy Tale Add-on Stories Pick a (prepared) beginning of an obscure fairy tale from a pile. Continue the story for about five minutes. Fold original beginning over and pass it on to the next person. The next person folds over everything but what they just wrote and continues. When you've received the story you started with, unfold it and read the entire thing. Then write an ending that wraps everything up somehow.
• Chronology Write a quick, simple story. Then write it backwards. Then write it out of order. This exercise is intended to improve skills on what to reveal when.
• Household object Find an everyday object sitting around your house. Then take ten minutes or so to write about that object in detail. Description is a good place to start, then texture, then the rest of your senses, then perhaps how it can be a metaphor.
• A Step-by-Step Story Have everyone write a setting on a piece of paper and pass it on to the next person. Then everyone describe a character and pass it on. Describe a second character and pass it on. Then write a climax (that may or may not use the characters) and pass it on. Then take the four elements of your sheet - all from different people - and write the story at least up till the climax.
• Something Specific Combine multiple elements to tell a story. For example, write a story where high fantasy characters (elves, dwarves, centaurs, etc) work as short order cooks where the following words must be included: sordid, green beans, bollocks, guinea pigs, ransom, and foamentation. The story has to end "...and the dish ran away with the spoon."
• Object Backstory Given an inanimate object with history, like a 5-dollar bill, a judge's gavel, a teddy bear, a reporter's microphone, write a backstory or a history of its travels.
• Boggle Sentences Using a Boggle board (or indeed anything where you can get a random selection of letters), shake it up and write down the first seven or eight letters. Think up a sentence where the beginning of each word begins with the next letter. For example: T E L C T N D could be "The elephant laughed 'cause tigers never dance."
• An Opposite Concept Everyone write down some words or concepts. Then put them in the center and choose one. Then spend some time writing about the opposite of that thing.
• Denial-Anger-Bargaining-Depression-Acceptance Think of something trivially annoying that could happen to you, like getting the wrong pizza or breaking a shoelace, then write an internal monologue, going through the five steps of grief in the process.

Side note on giving feedback: My rule of thumb for criticism is that for any short, on-the-spot writing, only positive feedback should be given. ("Yay for writing, no matter what!") For any serious writing done ahead of time, give constructive criticism in equal amounts positive and negative: say one thing that needs to be improved for every one thing you liked. That helps make the story better while keeping the author from getting too discouraged to continue.