Tons of these (such as the entire finale), but episodes 7 and 8 take the cake, with the rivalry between Lunar and Sun actually starting a war, which is resolved with a suitably epic sing-off. Then throw in Saru, who pulls off the most epic use of A Storm Is Comingever. The whole deal is topped off with the revelation that Maki was not attacks by Lunar's bodyguards. Maki attempted to attack them by launching herself will her shell gun, missed, and pancaked into a wall.
Mawari and the rest of the school's female population storm the school during Sun and Lunar's singing contest. They stop when Mawari sees a crazy fan.
Sun hitting Lunar and then Kai with volleyballs without even trying.
Sun imitating other people's laughs.
Furiyo dressing up as a human girl named "Brittany".
Sun and Lunar turning an all-out gang war into a dance party.
Nagasumi shifting into permanent Hokuto no Ken Mode at the end of the series.
One episode has Nagasumi drinking a potion to increase his popularity, causing all the girls in the series to see him as what they're ideal man would look like, including Mawari seeing him as a defenseless grade schooler and his mom just seeing Masa.
Sun agrees to pretend to be Lunar's boyfriend to help her prepare for a soap opera role. Cue her shifting into a more musclebound, macho persona and taking the term "beard" a little too literally.