This activity works best to practice a new sentence form that uses simple declarative sentences - something like past or future tense.
I start off the demonstration by writing a sentence on the board, like "I ate katsu-don last night." I say it to the JTE and tell them they can respond with the following options, which I write on the blackboard:
If I've successfully deceived the JTE (they thought I ate katsu-don last night, but I didn't!) then I say in an excited voice, "I got you!" and write it on the board. If they were correct, I say "That's right..." I write both those options on the board so the students can look at the flow of the dialog.
I do a few more examples of statements until the students understand the nature of the game. It helps to have the JTE explain what "I got you!" means and to act very animated when you've tricked the JTE.
Next I ask the students to think of their own sentence that will trick their friends. After giving them a minute, I tell them to say it to as many people as possible, and remember how many times they say "I got you!" After about 5 minutes or so, I call time and survey the class to find whoever said "I got you!" the most.