Divide the group in half, and have them line up facing each other with the facilitator at the head of the line.
Ask once side of the group to argue in favor of an issue and the other to argue opposed to an issue. After about 10 min (or when the groups run out of arguments) switch, and now they each have to take the opposite side of the issue.
Examples of good snap debate questions:
- we should have a policy prohibiting x - no one should be allowed to x - X is our responsibility