String of Pearls
The first performer to enter the stage walks downstage right and declares the first line of a story, for example "It was another awful day in the hell dimension.".
A second performer enters to downstage left and delivers a last line of a story that is as different as possible from that first line, for example "Finally, the accounts were balanced and the whole office could go home.".
Next someone comes centre stage and announces a middle point in the story, for example "The rabbit had eaten too much and felt quite ill.".
Now, in a completely random order, all other performers enter one at a time and add more and more lines to the story. They can go anywhere in the time line and add a unit of action. The whole story so far is recapped out loud each time a new line is added. The idea is that when the last performer enters and takes their place, the story makes complete sense from start to finish.
Instructions
It's much easier to go near the beginning than near the end. If you are one of the last performers to enter, think "What elements of this story haven't been explained?" and create a line that fixes one problem.
Think about the fact that "An improv show is a diamond" - once you are halfway through building the story, don't add any more elements. Just fix things.