Chain Sentence
Teams of two students orally construct the first sentence of an invented story. To orally make the sentence, each says one word, trusting their ears to recognize conventional grammar, until a long sentence evolves. Shape the improv by setting the tone of the sentence. Make the first sentence of:

a ghost story
pirate story
love story
mystery
any story, etc.
Beginnings:
This exercise can be used to generate the first sentence of a Chain Story where each participant adds a section to a tale.

Endings:
The chain sentence exercise could generate a "last sentence." This sentence is written on a piece of paper and placed in the middle of the story circle. The game is over when the story has woven around to the point where someone can say the "last sentence."

Describing a Stone
Pass a stone around a circle of students. Each student must say one word describing the stone without repeating what has been said. See how many times the stone can go around the circle without repeating words. Adjectives such as hard, smooth etc., are a start, but any word that comes to mind is acceptable as long as it is inspired by the stone. For example, a smooth, round, white, oval stone could suggest "egg."

Spontaneous Poetry
Four poets sit together. Each takes a turn spontaneously reciting an improvised poem after someone has "thrown" them a first line. The "poet" speaks the first line and leaps into improvisation at the end of the sentence. The poem does not need to rhyme. The poem must have a vivid image somewhere in it and a sense of finality, or closure, when it is done.