I should add, when I say "explaining how and why we use them" this should include a brainstorming section with your students, as to what they consider an impossible/improbable situation. I'd suggest:

Winning the lottery.
Getting your dream job.
Meeting a celebrity
Going on a date with Johnny Depp/Scarlett Johansson
Playing for your favourite sports team
Becoming a billionaire

There are lots and lots, but I'm sure you know that!

There are negative/depressing ones too, but I'd advise against them in a demo class unless your students suggest them during brainstorming!

To clarify my feeling on conversational classes (demo or otherwise), I don't think giving your students a topic and then expecting them to discuss it, while you correct their English, constitutes a class. It's a conversation they could have at an English corner at a local bar. If they go to classes, they should get a class: you should teach them new grammar and vocabulary and give them a chance to use the new grammar in reading, writing, listening and speaking tasks. Even at advanced level (C1-2) where discussions in class are longer, it should still be focused around the acquisition of new grammar or vocabulary, not around a topic of conversation.

Giving your students a subject and asking them to discuss it while you correct them, not only shows a lack of preparation on the part of the teacher, but also you risk alienating any shy members of the group who will not want to risk the embarrassment of being corrected, so will opt for a safer response rather than pushing themselves linguistically. Good scaffolding would help this, and conversational classes are rarely scaffolded well.