I love to do this activity when we have 5 minutes left before lunch. The kids get into pairs -one takes the pro side and one the con side and then I assign them a topic to debate.
The catch is they have to give a reason for their argument. That's actually the purpose of this exercise-it's critical thinking, but also being able to verbalize their reasoning. This is typically hard especially for the younger kiddos because they are used to just saying it's right, not necessarily why it's right. Each student talks for about 1 minute, the other gets a turn for 1 minute; and we come back together and share some of our insights.
A few topics we've used so far this year:
1) A shark would/would not make a good pet.
2) Girls should/should not be allowed to be President of the US.
3) Money is/isn't the most important thing.
4) Students should/should not be able to make their own rules.
5) Aliens really do/don't exist (my kids are obsessed with aliens this year, I don't know if they were watching Falling Skies all summer like I was or what :).
If you try this and it doesn't work at first, don't give up. It takes time for them to have the confidence even to think this way. But I really think it helps in the long run. They know how to justify their reasoning when it comes to writing or eventually open-ended test responses.