Classic cognitive neuroscience studies that highlight conclusions that could not be drawn from behavioral experiments
For introductory cognitive neuroscience, I think it is often best to start with interesting neuropsychological cases -- I think such cases provide an intriguing and intuitive way to get into the relationship between mind and brain. You already plan to talk about HM (and you can mention the movie Memento if you think students might be familiar or interested in it), I would suggest following that approach in the other domains. A few suggestions:


Executive functions/social behavior: Phineas Gage (frontal lobe damage produced impulsivity, rudeness, etc.)
Language: pure anomia (patients can report lots of information about an object but can't name it), jargon aphasia (patients produce nonsense words without realizing they are nonsense)
Conceptual knowledge: semantic dementia (progressive degeneration of semantic knowledge)
Spatial attention: hemispatial neglect (patients don't notice things on one side of visual field, usually the left side following right parietal lesions)
Motor control: ideational apraxia (impairment of action planning), ideomotor apraxia (impairment of object use), optic ataxia (impairment of visually-guided reaching)
Face perception / visual expertise: prosopagnosia (typically described as impairment of face perception, though it affects other versions of visual expertise)


More generally, there is a good two-volume set called Classic Cases in Neuropsychology (Vol. 1, Vol. 2) edited by Chris Code et al. that describes lots of interesting and influential cases.