Traditionally, Wallington have asked to imagine something similar to the comprehension piece and write about it, no choice. Sutton tend to give a "non story"-imagine you're prime minister for a day, what would you change or a letter as above with no choice. Wilson's have previously had a choice, so often something to suit everyone.
I found that boys struggled to write a story-they found it a good idea to spend a few minutes with a plan-who are the characters, think about a beginning, middle, end. I then encouraged them to throw in adjectives, adverbs and bonus points for metaphors/similes! Try and start with an interesting first sentence. Think what would make a teacher happy!
You can't actually do very much in 20 minutes. I think they are looking for ideas and reasonable use of language. They are also somewhat forgiving if you get completely the wrong end of the stick as one of mine did