My son went as a robot one year. We took a box and painted it white, cut arm holes & a hole for his head. We bought a length of cheap white dryer hose and made those to cover his arms & legs - we pinned them to his clothes with big safety pins and you couldn't see them once the box "body" was put on. We stuck all kinds of odds & ends on the front with a glue gun to look like controls - buttons, stray hardware, those little caps you put on the bottom of chair legs, even an old TV remote. He had fun collecting all the junk to stick on there - the basement and the "junk drawer" are gold mines! If you want, you can take another box and make a head covering for him. If you don't want to deal with paint, just cover the box with gray duct tape and use the metallic dryer hose.
For trick or treating, we hung a bag onto a cup hook stuck into the "body" -- he didn't even have to carry it! And you could stick on those glow sticks just before he goes out and/or use some reflective tape on the back. Safety and creativity at once!