Re: what is the THIS that you want teaching?
"How computers work, as opposed to how to use them."
OK, so are we taking an historical/hardware approach or building up from logic (and/or/not and Boolean algebra, leading to shift registers, half adders &c) or from the conceptual side (Von Neumann machine/Turing computability which rests on the 'diagonal proof' and its generalisation) or through programming (variables, assignment, loops, subroutines/functions then into more abstract areas) or all of those?
Could take a bit of time (and need some serious skills). Best of luck. One tiny activity I use sometimes: take an 8 by 8 grid of squares on squared paper. Draw a resonably complex shape (each square is either black or white).
Now devise a way of sending the shape to someone else using an sms message. Document the method for reconstructing the shape.
Now find a method that will work for a shape drawn on a 16 by 16 grid, and then a 32 by 32 grid &c.