For opposite day you could go to, like, walmart or target and buy a cheap dress and leggings (since i'm sure it's colder there) and since it's halloween time you could buy him a girl wig.

For the 80's, there should be children's costumes at walmart and such with the big sunglasses, bell bottoms and big afro wigs:)

That sounds like a lot of fun, we only ever did that in high school and it was SO funny seeing everyone's costumes.