At the end of that given time, stop the students and have them each pass their essay on to the next student. The next student has to read the composition given to him/her and then continues the new story with a sentence/time limit as before. This continues until their own paper returns to them.
This activity means that students are composing, reading material written by others, using their imagination, practicing writing clearly so others can read their writing, and of course, practicing their English. Students always love the final results when they receive their original paper back with a story developed by everyone.