The best part is Aang wouldn’t even teach him airbending in the beginning. First it’d be brewing the perfect cup of teach and blowing on it in juuuust the right way that it cools down ”But not too cold, or you’ll ruin it!”
Brushing Appa of course. Zuko would be pretty decent at that, but would come back to Aang absolutely covered in slobber and bison hair. “Awww! He likes you! <3″
Then it’d be calming meditation and slowing Zuko’s breathing (”But I am calm!”), tracking an elusive and nigh legendary animal so they can ride it, teaching patience by baking the perfect cake ”You just slapped the frosting on! It looks awful! *airbends it into the horizon* “Again!”
When Zuko finally gets it right, he almost smiles at Aang saying: “Excellent work, my student.” Then to Zuko’s consternation and horror Aang flings it at the head of the Fire Nation Governor during a political function. “And now we run!”
Pranks would be absolutely the thing that would bluescreen Zuko. Here’s an airbending master, over a century old, witnessed the genocide of his people, and he’s juggling, doing dances and tricks with Momo for the village kids, and plaguing the local aristocrats and military officers with flying cakes and whoopie cushions.
“How am I supposed to beat the Firelord with whoopie cushions?!”
And then, perfectly serious for the first time since Zuko had met him: “You don’t. The Avatar is about restoring balance. This training isn’t so you can fight the Firelord - it’s so you can stop a war.”
It takes a long time for Zuko to understand this. But the weird thing is: the townsfolk actually like Aang. Sure the aristocrats and crooked merchants and the officers don’t, but even the common Fire Nation infantry hide their chuckles with a cough when they see their commander’s fancy armor get covered in honey and feathers by “Crazy Aang,” again.
And Zuko realizes, at that moment, that he’s been having fun.
So, waking early, with no prompting from Aang, Zuko feeds Momo, brushes Appa, washes off the slobber, brews Aang his morning tea -perfectly cooled- and then proceeds to make a new, beautiful cake with no explanation.
“That looks wonderful, my student. Your best yet! Who were you planning to-”
Then Zuko smashes it in Aang’s face.
It’s the first time since coming out of the ice that Avatar Zuko laughs.