Persuasive writing can be fun!
In the assembly, I pull two teams of kids from the audience. One team gives me a type of a person, the other team provides a job, activity or hobby. When we combine them, we have a silly, impromptu topic to spur the audience to react to. In a recent school, the teams gave us astronauts and mining.
At this point I bring up three kids who can think of a reason why that would be a good idea, and three that can see reasons why not to let astronauts mine. After arranging itself with the strongest answer last, each team presents its reasons and the audience votes for or against the idea.
Then I take a reason given by the winning team and turn it into an illustration, drawn live on my computer and projected for all to see it take shape. During lunch, I finish the drawing and give it as a digital file to the school, along with a document summarizing the content from the assembly. The two files can be printed off on a two-side flier to go home with the kids. Here's the drawing I made for the assembly mentioned above: