Vinyl Scratch and Octavia's antics in the background of most of the chapters, which eventually gets them mentioned in Princess Luna's note to her sister:
Princess Luna: The ones known as Octavia and Vinyl Scratch are now invited to play at all major Canterlot events due to their skill, but they are forever forbidden from entering the royal labyrinth.
When Princess Celestia's guards wouldn't let Princess Luna in to see her in Post Nuptials, she used a mind-control spell on them. When the same situation occurs here, Celestia has made the guards resistant to the spell...so Luna just pins them to the ceiling instead.
Guard: That's it. I'm taking that transfer to Manehattan.
Rainbow Dash: By the way, I think I'll pass on that Daring Do fanzine from now on. Twilight: You didn't like it? Rainbow Dash: Well, those fanstories were lame. I mean, one of them is just every character whining that they didn't believe Daring that Derring's boyfriend was Ahuizotl in disguise. I mean, seriously, who would think that was entertaining?
Mumble's battle with Pinkie.
"That wasn't very nice. I mean, that could have really hurt! Not as much as it did for Mrs. Cake when the twins were born! I mean, ouch, seven hours in labor was hard, and she kept saying these funny words to Mr. Cake that I'm not allowed to repeat, and when she somehow got a hold of that scalpel... fortunately, Mr. Cake's scars have healed, and Nurse Redheart sees almost perfectly out of that eye now..."
The Running Gag of Pinkie insisting that Applejack and Twilight are in love. Eventually, this leads to Applejack causing a disruption that is heard in Canterlot.
Not simply insisting, but genuinely believing it. So much so she uses it as a reason to go to Fluttershy for advice.
Twilight and Rainbow Dash were worried about Spike and Scootaloo, respectively. Rarity was busy catching up on her order from Fancypants, and Applejack was grappling with her unrequited love for Twilight. That left Fluttershy.
Luna's reaction to finding Tiberius, and Celestia's confusion at it.
"I shall name you Tiberius, and you shall be my ickle diddy widdle Tibble-kins!"