Before you rent/hire/buy any costumes, be sure to read Disney's costume rules for Mickey's Halloween Party. The rules are posted on Disney's web page. Don't get something that you won't be able to wear to the party or that security will make you remove/confiscate. Better safe than sorry. And make sure that you get something comfortable and safe for wear all night, safe for getting in and out of rides quickly (rides have strict loading and unloading time limits -- if you take too long, you could shut down the whole ride), and for being weather appropriate (it can be really warm at night at the parties).