Ask a student, a volunteer hopefully, to leave the room. While that person is out of the room you and the rest of the class decide on something very unusual that could have happened while they were out of the room. A good example is two students get married, the OHP explodes, basically whatever the students can suggest.
Then, the person who has left the room comes back in and asks each student in turn only one question and the full question is 'What would you have done if this had happened?'
And each student in turn answers in a full sentence for example, 'If this had happened, I would have bought some flowers'.
Now, they mustn't mention the names of anyone involved because at the end the student who is guessing has to work out what happened to whom and, if they can't, you can go round again with new answers.
[As this is for speaking practice, the students should use the contracted form for the conditional grammar - 'If this'd happened, I'd 've bought some flowers.']