
There is an activity I do with kids (8-11) on my first lesson, which ultimately focuses on them drawing on a piece of paper a self-portrait, with the things they like, and answering to simple questions such as "what's your name?" and "what's your favourite toy?"
However one of the questions on the piece of paper is: "how tall are you?", which of course kids for the most part cannot answer.
Hence what you see on the "greenboard of the day": one by one the kids come to the board and I measure their height. I use a short classroom ruler, of maybe 30 centimeters, and a lot of addictions and subtractions, which I ask the students to calculate in my place.
At the end, when we finally have moved to the presentation of the next activity, the students have practiced numbers, calculations, the metric system... while enjoying a bit of celebrity status in coming to the board and having their name written on it, proof of the complicity between them and the teacher.
Naturally there is also a secret purpose to this, which is allowing yours truly, the teacher, to learn at least some of the names of the students on the first day.
When two of them are the same height I put them back to back and we all decide whether they're really the same height... what's most amazing to me is that never in this activity I've witnessed the slightest hint at competition to "be the tallest".
As to the mermaid and the name of the the local teacher above it... they're leftovers of the following activity and of the copious drawing it involved.
Category: whiteboards
I invented a tongue twister. Me. whiteboard of the day
There is one word that, in my experience, every student in former Soviet Union countries and that part of central Asia will mispronounce. It doesn't matter whether they are advanced or beginner, great at speaking or utterly laconic.
Each and all of them will pronounce the word "clothes" as /ˈkloʊðɪz/, only slightly differently from the word "closes" (/ˈkloʊsɪz/), as in "she closes". *
It is one of those pronunciation errors which seems to be so confidently imparted from the teachers in school, it is very hard to eradicate, especially in adult classes.
To try and overcome the issue, I got into the habit of systematically correcting this error while the students are speaking, in the hope that the feeling of annoyance caused by the interruption would crawl down into the secret part of their brain where the mistake hid.
Seeing that this wouldn't work, I started to periodically act a little skit in front of the door:
"I close the door", I would say, "she closes the door. (opening and closing the door like an idiot) Say like the first, not like the second!"
One day not long ago during one of my YL teenager classes I came up with a tongue twister in order to help against this pronunciation obstacle. It's on the whiteboard of that day:

As you can see the rest of the lesson traveled from fishing to Robinson Crusoe, from metal detectors to the present continuous. I don't entirely remember why. But I know that the "I scream you scream" pun I got from this:
* phonetic transcription provided by phonetizer.com (yes I cheated)
steps in a relationship whiteboard of the day

I'm sure not unlike many other teachers, I've drawn this a few times in class and it's always proved productive. At least is seems that this visual idea and the vocabulary in it are easily remembered.
Only once in my experience the addition about separation and divorce backfired, after I ventured those smiley faces on the celebrating divorcees' faces and made a couple of silly comments. This cooled off my small executive group rather awkwardly. To everyone's knowledge except mine, one of the members of the class was going through an unpleasant divorce.
Just a reminder a teacher should know enough about their students before venturing in such territory.
the American politics in class whiteboard of the day
The first time I expounded the topic (as a spin off intro to a Business lesson on Corporate Social Responsibility), I thought I would start from the elephant and the donkey. If anything, should the topic raise zero interest, this would qualify as an impromptu refurbishing of animal vocabulary.
Instead, as you can see, we went quite further, getting stranded somewhere between Reaganomics and Jimmy Carter.
In another lesson the same topic came up, but this time I started from an attempt to portrait the candidates, asking the students to identify them as I drew them. Trump's hair did it, if I remember correctly.
* The vocabulary you see around the characters below is the result of a few minutes of collective eliciting and sharing of ideas, and does not constitute representation of the teacher's political mind.
Afterwards almost everything was deleted, leaving only the two characters who at this point became just another husband and wife in a dreary living room. This allowed me to move on to another subject, "language to describe pictures", not after having elicited more vocabulary to describe the quality and color of such marriage, once established as a starting point that, not unlike a political candidate, the wife here appears to be "evasive" and "diplomatic".