You didn't say your reason for not wanting to be in costume ... if you don't want to go to the trouble and/or don't want to take up a lot of space in the suitcase, go to some place like Target and buy one of those headbands ... they have bunny ears and cat ears and devil ears and bug antennae... black T-shirt and black cat ear head band - presto - your'e a cat. Take off the headband and you're a person in a black t-shirt.
[Extraneous note: If you're the parent of a young child, go to the store the day after Halloween and stock up on the animal ears headbands at half price. The DAY WILL COME when your preschooler or kindergartener comes home and tells you that she needs a bunny costume TOMORROW ... you'll be all set!]