For my class today, I chose writing letters as my writing activity. It's a lost art these days, but those who participated in my class today had some funny ideas.
One idea was "Letters That Will Never Be Sent," pretending to write to someone such as a favorite author, film director, someone you once knew or some sort of role model. The other was writing a "Letter From a Place You've Never Been," imagining yourself on a journey somewhere you've never visited. I got these ideas from a book of writing ideas I've been using for my class. I also offered a variation of my own, using these same ideas imagining characters from books or fairy tales writing a letter to each other, or make up characters of their own for the same idea.
This may be fun to try once in a while if you need some writing ideas.