One of my favorite things is a cat...black pants/top/shoes or boots...add a store bought tail and some store bought ears...a bit of black eye liner for facial details and your done! Or, if you can find a face painter...have them do your face as a cat!
What about dressing up like the 50's? Long white button up t-shirt, rolled-up blue jeans, bobby socks, little white tennis shoes(or oxfords if you can find some!) and a scarf in your hair/around your neck? Chew a big wad of bubble gum too!
Google homemade costumes...there is a ton of stuff out there!
Have fun!!!