A couple of my kids have been writing a "choose your ending" story - where the characters have a couple of choices, and the reader decides which choice they choose. They have a number of places in their story where the reader gets to decide.
They found this very easy and fun to write, as it really let them be imaginative.
We have also written stories as a team - each person tells part of the story, then it goes on to the next person. This is really quite fun with a group of kids!
MUCH better than watching the boob tube!