Mystery key stories - each tag includes a writing prompt dealing with a key and is attached to a real key. Students randomly select a key and write a story. This blog post includes some great story lines to include on the tags!
Musical chairs writing. Fun idea! When the music stops you pick up writing where the last person left off. Time intervals baed on grade level. this gets the kids creative and is a good way to share each other's writing
Careful with this. You should definitely use other words for "said" in your narrative, but your dialogue tags (if needed) should be unobtrusive. "Said" is okay in most cases, and adverbs should be avoided (the dialogue by itself should convey the desired emotion).