In theatre, there is an exercise called "Scene Jam". It's an improvisation exercise where you focus on creating a scene (story) organically and see where the story ends up. I think it would be fun to do it as a writing exercise.
Rules:
- Don't Deny Someone's Reality. - Know what was written before and build off it. (Yes, and...)
- Be kind and respectful.
- Have Fun.
Opening Line:
Once upon a time, there was a young prince who lived in a shiny castle with his mother, father, and younger sister. Life was happy for the royal family, until one day, the princess was kidnappped by a dragon!