The cape and light-sabers complete the look. You probably will not be able to use the arms without getting the cape out of the way. It serves two purposes. It hides the unwieldy backpack and it discourages others from triggering the mechanism. I think it's an effective costume without the backpack so give it a try!
If you like this costume, please check out my other instructables and halloween builds at Creatrope Halloween
Some of them are in instructables as well.Godzilla / T-RexPVC Mannequin