Rikka uses the Konami Code on a soda machine (long story). She gets a free drink.
Rikka and her occasional Mundane Made Awesome moments (Someone flicks a bit of paper at her, she enters Bullet Time to snatch it between two fingers...and misses, the paper hitting her in the face; Performing elaborate arm gestures and hand signs in front of a train door, and to have it open in a burst of Dramatic Wind...and then calmly walk inside like nothing just happened.)
Sanae annoying Yuuta, deliberately putting emphasis on "Dark Flame Master" in each sentence and getting stuffed in a locker for it.
Any time Yuuta or Shinka bring up memories of their chuunibyou past and react in utter embarrassment.
Yuuta coming back home and finding his little sister and Touka Playing House. Let me explain: a 20-something girl and a kindergartener playing together and in the middle of a... divorce and custody agreement!
In episode 1 of Ren, chuunibyou!Kumin trying to attack Touka and failing miserably.
From the same episode, Shinka is a bit jealous that Sanae outscored her in tests, and gets to make the commencement speech to the new freshmen. So she flings a piece of paper, hitting Sanae in the forehead while she's up there. Sanae gets back at her by quoting an embarrassing line written by Shinka during her chuunibyo phase. It seems to work, as Shinka is visibly disturbed by it.
The end of Sanae's introduction in episode 3: twirling around and, after coming to a stop, getting tangled in her own hair.
Yuuta's reaction when Shinka attempts to take a picture of his magical girlcostume in episode 3 of Ren - "a flawless parabola" indeed!