The 750 words one month challenge
You have to write 750 words a day (not a month) for a whole month. If it was 750 words a month I might manage it. (link)
Write exactly 100 words a day for a month. (link)
I'm not sure which would be harder: writing an exact number of words would be challenging. Anyway, if you accept either of those missions, it should keep you busy till Halloween!