Simple, yet effective. With a few artistic talents and some old junk you likely have laying around the house anyway, you can create your own Nintendo Mii costume. This guy used some spare cardboard to fashion a tennis racket and little red rubber buffers as peg hands. The solid red t-shirt seals the deal.
I imagine the mask is simple enough to make in Photoshop or Illustrator. The faces themselves are rather basic, strictly geometric, so not much of an eye for art is needed. You might even be able to find a way to make a large Mii face online somehow, if there’s some sort of generator out there for making Mii faces. Other than that though, this seems like a simple, effective and geeky Halloween costume that you could make for next to nothing.