What are some super quick, but cool, costume ideas for a 4 year old boy?

I know we have lots of questions for last minute Halloween costumes, but this one is a bit different. Trick-or-treat is tomorrow, and my friend’s 4 year old son lost his costume. Or hid it. Or, who knows with a 4 year old.
So far we have a jellyfish, a bag of jellybeans, or a construction worker. None of which seem to be inspiring him. Any creative jellies with ideas?