Something that would simulate the distraction experienced by people with ADD. Like maybe playing a background recording of multiple people talking while asking them to read a passage and then perform some kind of test or quiz assessing reading comprehension, i.e. something that requires concentration. Or any kind of annoying or distracting stimulus. Just an idea, not sure if it's feasible. Or, possibly superimpose two different conversations in one audiotrack and ask them to pay attention to only one of them.