The grim reaper is a great scary costume idea that is easy to put together. Chances are, most people don't have a black hooded cloak at home. If handy with a needle and thread, purchase some inexpensive black cotton and make one at home. If not, look at a local thrift store or online to find a cheap one. Some old sheets could be dyed black and made into a fairly convincing costume with some safety pins in a pinch.
The scythe that the reaper carries can be made with a cardboard blade covered in aluminum foil.