Two truths and a lie is my go-to lesson after vacation, since it requires absolutely no prep and gets everyone to speak (and to practice the past tense).  One by one, they go to the board and write three sentences — two true ones about what they did over break, and one lie.   The class then votes on which statement is the lie, and the aim is to fool the class into picking incorrectly.

And yes, the title was a real submission.  Needless to say, some of the students are better at this game than others…