Write a story based on the end of story by another author. Read a favorite book or short story and think about the ending. Whether it was a good ending is irrelevant to this activity. Write an additional chapter to the story. Write it with an understanding of the character's personality and of the book's philosophy as a whole. Write it with a new take on a character. If the character was a villain, portray him as a good individual; think of new ways to craft the character. Develop the story where it stopped. Use the author's original voice or change it.
Write the same story in three different tenses. Develop a plot and a character. Write each version of the story on one page. Each version should contain a different tense: future, past and present. The narrator, for example, would tell her tale while experiencing it (the present), while remembering it (the past) or while envisioning it (the future).
Write the same story using different narrators. Each story can be one page long and must be identical; only the different narrators would provide differing opinions and insights. Start by writing in "first person" form. This would require the writer to write using "I" rather "he" or "she." For example, the narrator would say, "I went to store."
Then, write the story from the author's point of view---this is not a character in the story. This is the author's voice telling the author. For example, the narrator, which in this case is the author, would say, "She went to the store." This perspective is the "third person" point of view.
Award winning writer Ursula K. Le Guin recommends this activity in her book, "Steering The Craft." She believes writing in different voices can help an author develop her own.
Write a story only using dialogue. Instead of writing a scene or describing the settings and surroundings, have the characters do this. Have them describe the conflict, plot and idea of the story within a back-and-forth dialogue. For example, two characters can tell a story between themselves just by speaking to one another. This provides a lot of detail for the reader and allows the writer to focus on the characters' voices. This exercise helps the writer practice writing natural, colloquial dialogue.