Hi Luann,
Mickey's Not So Scary Halloween Party is a blast. Part of what makes it so entertaining is the costumes worn by other guests, children and adults. My estimate is that about 80% of guests wear some sort of costuming element. This might be just a silly hat or Halloween-themed tee, or it could be a full-on Haunted Mansion ghost costume for a group of 12 friends. Some costumes are store-bought and others are handmade. It's really up to you. If you look at this
Halloween-related post on the Disney Parks blog last year (click on the second gallery of dressed-up guests), you'll see just a few of the many types of costumes guests wear at the Magic Kingdom during Halloween events.
There are a few rules which will be stated on your party ticket. Guests can't wear full face masks. You can't wear wings, tails or anything else that drags on the ground (so you don't trip people). You can't wear graphic, bloody, or offensive costumes (it's NOT so scary), and you can't dress in a costume that looks so much like a Disney character that you could be mistaken for the real character.
I will tell you that Florida in the fall can still be quite warm. You'll want to avoid heavy or furry costumes for the sake of comfort. If you want to keep it simple, wear a jersey and cap from your favorite sports team to be a ballplayer, or wear a Hawaiian-style shirt and a lei to be a hula dancer. Or just wear a tee a with a Jack-O-Lantern image and you'll be good to go. On the other hand, feel free to create something as elaborate as your imagination.
Candy bags are distributed at the party. They're approximately gallon-size sturdy plastic with handles. These have always worked for me, but you're welcome to bring your own bag if that suits you better.
Please let us know if you have more questions. We're happy to help.