We had a really fun improv practice at the Toastmasters dinner today: people sit around the table and build up a story word by word -- each person says one word. At the end, someone can say "the moral of the story is", and everyone continues to wrap up the story word by word. It works best for a small group, e.g. 6 people. It's easy to get lost in a bigger group.
Here is one short story we built, word by word (I can't remember exactly what it was, just to give you an example):
Once, upon, a, time, there, is, a, grasshopper, on, the, desk, and, then, it, fell, onto, my, lap.
And the story goes on and on. It's a fun process that gets everyone involved in storytelling.