Their faces. All of their faces. Especially Shizuku's face whenever Haru does something unexpected.
Particularly, whenever Shizuku's face turns red. It's not just a blush, it turns entirely, completely, red. Usually combined with a spectacular expression.
Speaking of something unexpected; Shizuku and Haru's first kiss.
And Shizuku trying to improvise Brain Bleach afterwards by 'eating her brains out' with Monja. It fails spectacularly.
Shizuku's realization that she's in love with Haru. After making five consecutively more hilarious faces at the revelation, she decides;
Shizuku: This could be difficult.
Haru tries to bring the rooster to school by hiding it under his shirt.
Natsume's introduction. From the garbage, to the Death Glares from both Haru and his rooster, to the explanation that the reason she has no friends is because she's So Beautiful, It's a Curse. After she threw up. With garbage still on her face.
The entire chicken coop plot. First, against all reason, the school decides to let them build a chicken coop. Then Shizuku comes in concerned that she's late, when everybody else is concerned with her 'granny clothes'. Then when they get to the store, Haru gets into an argument about a chicken house, and when he decides they can get one on mortgage, the objection raised is 'but chickens don't earn money!'. Then Haru's old friends show up, and after the slowest chase scene imaginable, they realize that Shizuku is in love with Haru. Shizuku pays them back by tricking them into building the coop.
Also, Mitsuyoshi is totally cool with giving Haru a chainsaw.
Haru decides to stop glaring at people and smiles at them instead. The school's entire female body immediately falls head over heels for him.
"Did you do it?"
Haru: DID YOU SMASH THE WATERMELON TOGETHER!?
Haru getting a whiteboard on which to explain his plans to get Oshima some friends.
Oshima isn't 'timid', mind you... she's just five different words that mean exactly the same as 'timid'.
Thanks to Haru and Shizuku's usual obliviousness, they get into a relationship talk with Oshima right there in between them. Oshima clearly doesn't understand anything about the way the two of them thinks, and comes to the conclusion that Haru is clueless about Shizuku's feelings. She asks him if he has any balls. Dramatically. Everybody stares at her. She runs.
Natsume desperately wishes Banana juice could get her drunk.
Natsume: BANANA JUICE WON'T GET ME DRUNK!!!
It makes sense in context:
Shizuku: You look like you're enjoying yourself.
Haru: Yep. I was thinking about a dead person.
When Haru licks Shizuku's hand. In the middle of a busy hallway. And they get into a quarrel of love without noticing all of the STARES.
Shizuku treating her dad like a child when his store goes under and he doesn't tell her for two weeks. She immediately calls her mother, and relays her mother's response in a perfect imitation of her; "Die Stubby."
This is the sixth time his business has gone under.
Haru is completely awed by Shizuku's performance.
Natsume getting so flustered, that she begins to read a book upside down. The English subtitles dub the book yrotsiH esenapaJ.
Shizuku comes in the middle of this conversation, where Haru is happily explaining that he was fine with where things were because a month ago Shizuku told him she'd think about their relationship. Shizuku begins freaking out (as much as it's possible for Shizuku to freak out) because she forgot to think about it. For a month.
All of episode 10.
Alright. Are you ready for a massive, massive, massive Spoiler? The mysterious Summer X is, in fact: Natsume. Who is also a chicken.
"Haru-kun! Don't hurt Santa!"
He deserves it. He's a fairy, and his reindeer were all naughty children in a previous life.
Haru and Shizuku both agree that Natsume is the strange one in their group of friends.
Oshima is still not used to team Haru's antics. So she's not sure whether two boys planning to kill each other for her means it's a good time to play a puzzle book with Summer X.
Haru tries to go to Cram school to keep up with his rival.
Sasayen knows Natsume loves Mitsuyoshi. He has known for a long while. He doesn't think it's that big a thing. Then Natsume figures out that he knows. The results... are faces you will remember for a life time.
Her reaction is then cut short to Haru coming into the picture. He's absolutely beaming, and sparkling more than Edward Cullen.
Natsume: You look happy, Haru.
Haru: Do I?
Sasayen: Did something good happen? Were you stalking Mizutani-san again today?
And for the rest of the episode, everybody sparkles. Even Shizuku. Haru practically fireworks, and Natsume sparkles just from answering a phone.
The fight at the festival. Shizuku and Yamaken decide they don't like pain, so they just ignore the fight and by some squid-on-a-stick. Shizuku scolds Yamaken for cutting in line. Natsume finally manages to drag Shizuku back to help stop Haru from fighting, and tells Yamaken he'll never win Shizuku's heart by buying her squid-on-a-stick. Yamaken tries to imagine flirting with Shizuku the way Haru flirts with Shizuku, and he needs some Brain Bleach now.
The shame girl is back, and she's mocking the cast for buying 'accurate' love fortunes. Natsume's love fortune explains that their true love is 'right next to you'. Neither of her love interests are present.
Mitsuyoshi: Hey Haru! No jumping off the roof again, okay!
Mitsuyoshi's blank stare at Natsume after she proposes to him.
Shizuku getting technical about the first sunrise of the year. Because if you're going to watch the first sunrise of the year, you need to lay down some ground rules. The sun is clearly copping out.
And when Sasayen looks down on Natsume, the rooster is right behind him. Staring.
Shizuku is in her room eating breakfast. Haru barges in, wearing what is apparently a miner's outfit and a butterfly net. After making a few nonsensical statements, he leaves.
Haru: You'll get fat if you keep eating Ramen!! Bye Shizuku!
Shizuku's only question about this, is why he's in such a rush.
Then he goes to Natsume.
Haru: You don't understand?
Haru: You don't understand?
Natsume: I don't understand!
Haru: It should be obvious right away!
Natsume: I have no idea what you're talking about!
And when he leaves, he tells her that her blog has typos. A lot of them. And when he tells Sasayen, he admonishes him to handle his bat properly.
The shame girl meets the Nagoya. They become friends.
When Haru and Shizuku first meet. Haru escapes by jumping out the window, followed by kidnapping Shizuku outside. This turns into a running gag when Shizuku is constantly getting kidnapped, and Haru is constantly fleeing the scene via window.
Followed by Yamaken's running gag of getting lost and emerging from bushes.
Practically the entire OAD. Especially the ending. Without getting too spoiler-y: DNA Testing. In the Edo period.