Demos on kindergarten kids are soooo easy too. Just get a few pictures, teach em what the things are in the picture and then play a game. Get em all standing in a straight line and ask em..."what's this" The first one to correctly say what it is can step forward. Keep asking until one is at the board, or front of the class. They love it and it's something that involves em all. Give the slower ones a little "leg up" by letting em advance a step or 2 as well so's to build up their confidence. Don't let em advance too much though, or they'll get lazy and think that you'll let em cruise through it always. The winner has to earn it. Takes no prep time, it's easy and fun for the kids and they'll love you for it.
Just another idea.
With High school and adult students, a Q & A session is good. Get em to write a question down, but time them...only one minute for 1 question. They must ask their question, you'll answer it and possibly ask them a question. The same question cannot be asked twice so it makes em practice listening, thinking and speaking too. It's also a good way to gauge what level they're at.
Works for me, might seem lazy, but impromptu demos are kinda hard to plan for so I just use these ones as my templates and fit them to the situation(s) at hand. Who says that teaching has to be hard work huh?
My dollars worth, peace y'all,
