Role Swap! Or how my mind interpreted it: body swap AU. Katara having to be the Avatar in Aangs Body and Aang being Master Katara
The two wake up being each others bodies. It takes a while until one realizes something is wrong - It's Katara that screams at seeing herself waking up next to her. Aang, despite freaked out, stays calm stating things will just turn out fine. He'll figure out something.
Then Katara remarks that technically, he's not the Avatar right now. Aang is stunned, realizes it's true and Katara begins to giggle. (Katara is lowkey excited to be the Avatar for the day, even though she got a lot to do herself and this doesn't fit with her idea of what to do today)
The two agree to spend a day like that, while Aang thinks about *something*.
Aang, as Master Katara, goes ahead and meet with the young water benders of the Southern Water Tribe. His first problem is the temperature. "Oh right, it's cold out here. How does Katara manage without airbending??" He brings the mittens along that Katara made for him, only to realize they are too big for Katara's hands.
(he has problems bending with those big mittens, but they are from Katara!... Kya runs to their house and brings Mommy Aang some that fit. Good girl)
His next problem arises when he actually has to teach water bending to the kids. He has never taught anyone any bending, he has only been taught, he's the avatar after all.
Lil Kya even points out "Sifu Mommy is bending weirdly today." and the other kids agree that it just doesn't look like Master Katara's bending. Aang is both surprised that they could tell (and proud of baby Kya. His little girl) and a little embarrassed. He keeps on trying his best tho!
Katara meanwhile on her way to Avatar duties, she took Bumi with her. After all, the two have agreed the past day that Aang would take Bumi with him today and Bumi was looking forward to it!!! Bumi was a handful, but Katara didn't want to disappoint him and thus didn't change the plans.
Katara has a surprisingly easy time talking with the heads of the Southern Water Tribe. (Let's assume they got business with the Avatar. I did not think about what. But let's assume it's something majorly important) Tho some of them remarked that the Avatar behaved un-Avatar-like, letting Katara realize she's been very much herself now- not trying to act like Aang.
She tries again, more the Aang way- no one says any further, but she's sure she failed the Aaang Way.
Later she runs into some kids, who ask Katara!Aang to show them some tricks! Gladly Katara shows them some waterbending tricks but the kids want to see the other elements
"Yeah, daddy! Let's firebend." - puppy eyed Bumi.
Katara freezes for a second. One of the kids asks her to go in the Avatar state. Another kid asks for the scooter and the third wants to see the airbending trick again. But Katara doesn't know how to do these things. She tries but fails. At least the kids laugh at that.
Her thoughts are distracted for a bit, wondering if Aang goofs around a lot during the day, until Bumi remarks, "Come on daddy! You're weird today."
(Also Bumi being wild and free, giving Katara a hard time. She had to be 'Fun Aang' after all, not 'Strict Katara')
Back home, Katara and Aang talk. Aang tells her what amazing waterbending master she is. The kids knew so much already, he hardly knew what to teach them. He also wonders about Kya, if she maybe has caught up to them.
Katara thanks Aang for the compliment, but has to give it back. Maybe he's a little too soft with official matters, but even then people listen to him and take his peaceful advices into consideration. She had known that before, but seeing the difference in person now, made her appreciate it a lot more.
She also tells him to goof off less during work- but she understands how nice it is to entertain the children.
"Bumi really seems to like firebending too." - Katara
"I know right? I wonder where he got that from, I hardly Firebend, unless I have to." - Aang
The two then embrace.
And the next morning it's all normal again for this short AU HC session. Magic, poof. Let's say the spirits wanted to allow Kataang to get each other known even better or something.