On learning what the traffic signals mean. "Red means stop. Green means go. Yellow means go very fast."
The starman's attempt to smoke a cigarette.
Starman's reaction to dessert. His face scrunches in absolute ecstasy (ooh, sugar!) and it almost in tears with how it tastes.
This leads to a nice moment later in the film when Jenny is describing how wonderful a honeymoon can be; after thinking a bit on how to put it so Starman will understand, she says it's "better than Dutch apple pie." Starman nods immediately, understanding without any problem at all.
Similarly, after having sex for the first time (ooh, sugar!), all he does is wear a goofy grin on his face for some time after. Granted, many guys have that reaction after good sex, especially if they're a virgin.