Sometimes I student comes into your office with a problem needing discussing and that time you were going to use to get ready for class gets used up. Helping a student with a serious problem is, sometimes, more important than preparing for class, in my book.
I know teachers who have an open discussion day. I also read about a teacher who had an open discussion day, and the students wanted to talk about sex. At the end of the semester he got fired. This would not happen at my university, I don't think. He worked as a telegraph clerk and finally in a couple of years found another job. This was in a freshman English class.
What do you do when you're totally unpreparred? (1) read some of your own writing and talk about your writing process (2) put chairs in a circle and go around the room and have everyone tell what they thought was good, or bad, technically, about the reading assignment for the day, (3) go around the room and have everyone tell why they want to write (if they didn't take CW to avoid technical writing), and what kind of writing they hope to do. (I've only done #2 of my list)
I carry maybe a 100 exercises in my head. What do you do? Any ideas? Please share them. Of course, we're all for being prepared. It's rather nerve wracking to go to class unprepared, but with time you learn that sometimes it can't be helped, and you deal with it. The students enjoy the sudden change of pace and often remember these classes the best.