Rei opening the refrigerator in one episode to find Professor Kenjiro's deserted body sitting inside, his face still the Oh Crap expression it last had, cradling a watermelon in his lap. She understandably freaks.
When the girls force their way down to see Rei, Kenjiro is unsurprised by their sudden appearance, in fact stating that he was expecting Akane sooner.
Rei's parakeet greeting her with "Good morning!" when it's not morning, and her trying to tell it that.
Kenjiro's bombastic greeting when Akane 'disturbs his work' in the first episode, her playing along, and them finally hugging while Momo wishes they didn't have to do this every day.
In Episode 3, when Wakaba is chasing Akane, the pair knock over a teacher carrying a stack of boxes. They both briefly abandon the chase to go back and help, the teacher admonishes them for running in the corridors...and then they resume the chase at walking pace, gradually building back up to a run. It's a lot funnier than it sounds.
From the same episode, Wakaba's letter of challenge is, on the outside, indistinguishable from a love letter.
Aoi: What a beautiful envelope. Momo: It's so... (sees "Letter of Challenge" on the back)...cute?