The Songwriting Scene blog has issued a seasonal songwriting challenge. Write a seasonal or holiday song, and compare your results with other songwriters who took up the challenge. See the details at Songwriting Scene.
It would be ironic for me to enter this challenge personally—I've written so many holiday songs, it might as well be my day job. But this is a good kind of exercise, because you always learn something by seeing how other songwriters responded to the same assignment.