First of all, if you're the one picking the topic of discussion, pick something that nobody can possibly argue against and still be considered a good person. For example, if you're in a college discussion section, or a high school speech and debate team, pick a topic like "Hitler was bad" or "Premeditated murder is wrong." It's always fun to watch people struggle with answering your claims. Especially if those people usually consider themselves to be "moral." If someone tries to tell you that it's generally accepted that Hitler was bad, and that there's no need to debate the issue, then call them an anti-semitic racist and/or a murderer. See what they have to say about that.

Now, you do have to be careful about what topics you choose. Especially when you're going up against a philosopher, since, as we all know, philosophers are devious, treacherous, and not to be trusted under any circumstances. You might think that "The moon is a spherical object rotating the earth" is a pretty good argument for these purposes, as science has proven this to be true. However, the more mischievous philosophers will be able to find something to argue against, even if it comes down to an argument along the lines of, "How do you know the moon--or yourself, for that matter--even exists?"

One way to help make the topics you pick more safe is by picking things that are morally accepted in our society. I doubt anyone will want to dispute that ritual sacrifice of virgins over a volcano is wrong, so that would be a perfect topic to discuss. Same thing with public urination; nobody wants to take the "pro" side of that argument, so it's much more fun when they have to.

In closing, be creative, have fun, and make sure you make the other person as uncomfortable as possible. Pick a topic and go nuts!