My cooperating teacher has had some trouble the past two days because she allowed them extended class time to prepare for a test. The students did not take advantage of this time and chose instead to talk excessively and disrupt the class. Class time will run much more smoothly if teachers always have something extra planned. One idea is to simply have a supply of higher order thinking questions to throw at them if there is extra time at the end of class. Just keeping them thinking will make for better classroom management.
Bored students are disruptive students.
I would add, though, that just having an activity on hand isn't enough. I've seen teachers hand out a worksheet for students to work on in class. The problem is that the worksheet is incredibly dull. The students ended up not working on the assignment and talking, etc. instead. Courtney, I like your idea about having higher order questions on hand.