Before you start you need to find out what they already know - so you don't go in and teach that.
Pick a new grammatical structure to teach them, and use familiar vocabulary - that way you won't be teaching them all new stuff in a short demo. My approach would be to have the students using the new structure by the end of the demo - to show that you get results and the pupils enjoy it.
If you have beginners then I would teach a handful of new words or very short sentences, such as "I like butter". If the goal is to have the students using the language by the time the demo is up then you don't want to be over ambitious.
I would do this using games - now my experience is with beginners to intermediates, and mostly children. That's no doubt why I find games to be the best method of teaching - and especially in terms of interacting well with the class. (The pupils love you.)
Also, if you are going in with games and activities, then have a few spare ones under your belt so you can drop what you are doing if you see it isn't working.
I should also say that my approach is totally interactive as I believe in people being able to speak a language when they learn it, and not just be able to write. However if the place where you are doing a demo is really into silent lessons where students quietly fill out worksheets then they wouldn't like my lessons for sure. So you had better check that out before you prepare too.
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