You can fuse Nightopians with the Nightmaren enemies to create a special kind of Nightopian. The more Nightopians you have in the level, the "happier" it'll be and the music will change to be more exciting or happy, or if you kill them, the level will have more calm, "sadder" music. The "sad" music just sounds kinda "eh, whatever" in terms of emotions. Not really happy but not really sad either.
There is platforming to be done as the kids. Generally you want to go straight for NiGHTs if you're trying to get good scores, but you can explore the level and look at things because there are some things only the kids can do iirc. Think of it as a pointless bonus thing that can be done for the sake of fun.
The game is a score-attack arcade-style game of sorts and it has a lot of replayability because of that. Try and get the highest scores by collecting a lot of things and getting high links to improve your ranks.
You can do paraloops around enemies and objects to collect/defeat multiples at once, and the l and r triggers are used to do various tricks as you around in the air. It feels good to be as graceful as possible and mix the little moves and dances in with your flight in the game.
You gotta use the boost thing to go fast and remember the levels, and collect a lot of things. When you get the Ideya, a score multiplier activates. If you don't bring it back to the main little thing you normally bring it to and fly over it instead, the level "resets" and all of the collectables and rings respawn again and you can continue to collect them as long as you like. Make sure to bring the Ideya to it before the timer runs out though, because if you don't you'll turn back to a kid and your score and time will be completely destroyed for the most part. It's sort of like a bonus thing where you get to do as many laps as you can do within the time limit to get more links and score and the like.
Another part of the replayability comes from the fact that the games music and atmosphere is so beautiful. For me, I love to boot up the game when I just wanna relax and have a good time as it always cheers me up and puts a smile on my face. The surrealist style is beautiful and the game has a decent variety of environments, and all of the dream worlds, Nightmaren, and even bosses are all related to the subconscious of the current playable childs' subconscious and psyche. The Puffy fight is kind of like trying to get a basketball in a hoop, and struggling to do so, much like Elliot was having nightmares due to the upcoming basketball game he had.
It's a great game and a cult classic, but if you're not one to stick around and enjoy the freeflowing, feel-good gameplay, then it's probably not one for you. It is kind of like Sonic in the fact that the gameplay feels flowy and good, though.