Piano and ATLA you've already sold me
And I do recognize quite a bit (rough timestamps):
0:20 this motif often comes up when something mystical happens, especially related to the avatar spirit.
0:40 this is the "something epic is going down" music, which is often used during the climax of many episodes (like, imagine Aang going into avatar state). I love it so much. It's also reprised at the end of the opening.
1:55 that's a motif they often use during denouement at the end of the episode, it's one of my favorites.
3:00 I recognize that piece but can't really manage to place it. It sounds sinister and evokes fire nation, but I could be wrong. Or Wan Shi Tong?
4:20 I'm pretty sure that is at least played during the happy ending of the finale (it references some motifs of number 2 on this list btw), maybe also in other places.
5:05 is of course the song that Iroh sings at the end of Iroh's story in Tales of Ba Sing Se (young soldier boy?)
But I admit that it isn't very easy, the piano being such a different instrument from what the ATLA soundtrack uses. Now I'm probably off looking for sheet music.
They have the sheet music link in caption haha. So you play piano... classic, 'pop' or jazz?