Here's another group activity that my class enjoyed.
* Start out by writing a simple sentence on the board
* Have the class write the sentence in their journals. (The above is an example.)
* Have the class come up with a word they can add to the sentence, and make sense. (I once had a child suggest, "I see four cats." Although the sentence is correct, the original uses the singular "cat", so the other sentences have to keep the singular version.)
* As they come up with a new word to add, write the revised sentence on the board and have them copy it.
You can take this as far as you wish or have time for.