In all seriousness maybe different written random symbols like Zener cards?
Be like “oh look, add one to star and you get wavy lines but oh shit you add one to wavy lines and you gotta bring another card in” and later “but what if we wrote the star as (dramatic pause) 1”
(This may be the card sorting exercise you referred to earlier, in that case ignore me.)
But the distinction between sign-and-signified is the single biggest aha moment you can get in CS, IMHO.