Hi, Lisa! I absolutely love Mickey's Not So Scary Halloween Party, and I can't wait each year to see the costumes everyone chooses! It's one of the best parts!
The only real costume guidelines are that your costume not be too gruesome or scary (it is, after all, the NOT So Scary party!), and they ask that your costume not include a face mask. You can use face paint or glitter or something along those lines, but no masks that conceal your face.
You can absolutely go dressed as a Disney character - and chances are, you will see quite a few characters wandering about! Popular costumes each year seem to be pirates and princesses.
Don't forget - grown ups can trick or treat too!
I hope your vacation to Walt Disney World is great fun, and I hope you truly enjoy the party!