You could describe a character’s adult life and the choices they made.
For example, you could build up the story of a character who went to university to study medicine, then went on to work in a hospital where he met and fell in love with Janice. You can continue the story from there.
As a character, you can take somebody the students know well, e.g. a character in a film, somebody in their course book. You can also build up the character with the class using a photograph as a prompt.
When you have a fairly comprehensive life story, you can go back to the beginning and look at alternatives. For example:
“If he hadn’t gone to university, he would have worked in his father’s firm”.
“If he hadn’t studied medicine, he would have studied accountancy / he wouldn’t have worked in the hospital.
“If he hadn’t joined the hospital, he might not have met Janice.
After this, students can write down some major elements of their own lives and speculate on alternatives using the third conditional.
Good luck for your lesson,