When I was a kid I made robot costumes from cardboard boxes - one large box for the body, a smaller box for the head secured to the top with duct tape (only tape inside because duct tape doesn't take paint). It's better if you can get an odd-shaped box for the head, like maybe something round - clear plastic pretzel barrels might do, if you can find one big enough.
Cutting holes in the boxes for arms and faceplace is easy enough with garden shears or utility knives. Some smaller holes in the head can be cut on the sides and back of the head and covered with speaker cloth; these look like "ears" and provide both sound input and air circulation.
For arms, I always used a length of flexible dryer vent tubing. It gave me a Robbie-the-Robot look and was simple to cut to length and secure to the box with zip ties and duct tape (tape the inside well to smooth out the rough edges or it will be uncomfortable for the kid). In my day, dryer tubes only came in white plastic, but now they also come in silver foil, which would look much better on a robot costume. A pair of gloves of some type finishes the effect.
I usually layered the boxes with paper mache for added stiffness, but this is optional. The paper mache does take paint a little better than bare cardboard, though.
I usually painted the costumes with interor white latex, then added detailing with colored model paints. Hint: outline the details lightly in pencil first, then paint by hand. Use pics of R2 units from the Star Wars movies for detail ideas. Also use reflective tape and decals for better visibility when trick-or-treating. Colored electrical tapes also work well, and for some details I used duct tape.
An easy additional detail was adding a flashlight to the costume. You can find cheap lights in WalMart or even Dollar Tree that you can tape into place in a pre-cut hole in the box, allowing the kid inside to turn it on and off. Also look for battery-powered Christmas lights to give a few added detail lights to the costume.
One year I also added a fold-out tray to the front of my costume. Just cut a rectagle out of the front, but leave the bottom edge attached. Add side and rear walls to it, and it will pivot our like a mailbox slot. Makes a great place to put candy, but tell the kid to be careful when running, the candy bounces out.
Unfortunately, I don't have any pics of these costumes to illustrate. I think my parents might have some but I don't have them myself.
