I had to prepear many demo lessons because I constantly lost my jobs and had to apply for new ones. What I also learnt in my 20 years of teaching career is that games and game-like activities are the most powerful means of teachers to gain the attention of students and to provide a high level of involvement and practise. Apart from that, there is also a strong recommendation of the 'ludic approach' in the Common European Framework of Reference for Languages.
So what I suggest when planning a demo lesson: Prepare 5 minutes introduction of an item - letīs say should or shouldnīt, comparison, past tense - just anything relevant on the A2-B1 level. You can do it by powerpoint representation or neatly prepared handouts. The presentation or hand-out can consist of either a short text and some gap sentences which make the students find the rule of how to use the structure e.g.: We use any/some with uncountable nouns. We use some/any in questions etc. Or present some model sentences illustrated by pictures and add some fill-in-the-gap-sentences with a clear rubrics.
Then use a game to get the whole audience involved and to practise the structure. I have found this source when preparing for a show lessons very useful: ESL for Adults - Activities and Games to Make Learning Easy
(it's also in paperback http://www.amazon.com/ESL-Classroom-.../dp/1478213795
). You can use e.g. activities: 'Decision Time' for practising the conditional, 'Comparative to get in Order' to practise the comparative, 'Alibi' to practise past simple questions, 'Guess the Action for present or past continuous', 'Grammar Knock Out' for any other structure. Plan the activities in groups of four or five. Demonstrate the action with one or two particpiants (2 minutes) and leave 8 minutes for intensive practise of course participants. After the activity is over, give a short feed-back on mistakes participants made and ask them whether they liked the game. When asked why you chose a game to practise the structures you can hint at the high amount of practise and students' involvment they provide and the fact that you believe that language structures must be practised orally several times before the students achieve accuracy.