You could go as a classic superhero like Superman and wear a blue suit with a red cape and boots.
Well, if you want to be a princess, a long, flowing gown with a tiara would be perfect. Maybe add some glittery jewelry for an extra touch. Another option is to dress up as a witch with a black dress, a pointy hat, and a broomstick.
For fictional character day, you could choose to be Harry Potter and wear a robe, a wand, and round glasses. Or, if you prefer something from a fairytale, like Cinderella, a big poofy dress and glass slippers would do the trick. It really depends on your favorite characters and your style!