There are some great ideas about writing on busyteacher.org. I used them as speaking activities in my class and they worked well. If you have 5 minutes or if the students get bored, you might want to use them!
In the Busy Teacher Facebook page, there is a Creative Writing Prompts photo album, with lots of great visuals to go along with the prompts.
The actual BusyTeacher website also has 350 creative writing prompts as well, displayed in sets of fifty. Here is an example from one of the sets: