While much emphasis
is should be given to listening and reading for gist, summarising is often neglected, with writing often being limited to written exercises that are, more likely than not, cloze tests, single sentences and/or full-fledged essays.
Effective summary writing involves clear comprehensive skills and the ability to transform an original version into the students' own words, whilst, ideally, making use of any keywords they've picked up from the reading/listening exercise.
Apart from the obvious choices of summarising newspaper articles, books (short stories) and films, other options include using speeches, dialogues and even dictations. It can even be developed as a group activity, with students competing with each other to produce more concise, yet informative, summaries, combining their collective knowledge in finding the best way of putting it.
In terms of difficulty, along with technical writing, good summary writing is most definitely a challenge which helps students use real language actively.