I'm sorry, I should have seen that coming.
I can give two examples not connected with ESL. With an audience of 20, a speaker failed to turn up. Peter came to the rescue and spoke, without any visual aids, for 50 minutes, about the Duke of Wellington. (The talk was given on what was part of Wellington's estate, so local interest). He barely mentioned Waterloo, the Peninsular War, or the Duke's political career. He spoke about Wellington's views on agriculture and the long term care of estate workers. He was brilliant. Even those who thought they were well read on Wellington learned a lot.
In college, a professor was taken ill. The stand-in spoke about the mass movement of peoples before 1000 BC. It was not connected to our curriculum. We requested he come back, but sadly ........ But I still remember a lot of what he said, despite 50 years passing.
And now for a bad example. In India, without notice, I was presented with a group of smiling faces in a Himalayan school. They ranged in age from 5 to 14. I wished I had had something prepared. Sadly, the best I could do was show the children a new plastic 5 pound note, that you could crumple and yet it would return to its original form. And that led to why the English eat fish and chips. They were, I'm glad to say, very kind.
The main reason I want to prepare myself for an ad-hoc ESL related lesson/talk. is because in the summer we get visits from young people who are learning English (not together). We often go by train to Seville (1 hour and 10 minutes). They've all seen orange trees before. What an opportunity.
I do have one topic that has been suggested: A brief introduction to Blake, Thackeray, and Scott through quotations. We will discuss how they did, or appear to have, broken grammar rules (based on the use of relative pronouns). Discuss how grammar rules relate to the development of the language.
The point is that it is a total change from their normal lessons. It can be witty. It introduces some new vocabulary. It is always to hand.