On the teacher's in my school students and teachers switch roles. The students who have a higher level in the subject go to give a lesson in the groups with the lower level.
On that day I was given a group with the beginning level in English. I was supposed to train them in the past tenses. 
So, I added a bit creativity: me with my English teacher came up with a scenario. I acted out as if their teacher had been kidnapped by aliens and taken out to another planet. The class was at risk of cancellation and I was a representative of the other civilisation who had to save their class from missing. 
It was the whole improvisation for even though the group were just beginners, they actively involved into the action and started to ask questions on how their teacher got kidnapped, what was the name of the planet, how far it's located from the Earth and so on. 
Thus the mission of the lessons was completed. My students did use the past tense in their speech. 
  Though, I have to say that organising lessons from scratch is hard, no matter how good  you are familiar with the topic, not everything goes smoothly.  I remember the experience with that History teacher who got lost in the class, but not once was I in his shoes either. 
 Last year we did a lot of reconstructions of the historical events: war battles, political summits, political elections, theatre plays. I got stuck many times right in the middle because I couldn't formulate what I was supposed to say in a clear way.  
Sometimes, I simply didn't understand what I was saying. Thus the historical summits used to stagnate. 
In the beginning I felt like sinking through the floor when it happened. Then I saw that no one cares and freely failed.