Bit out of date now, but my son once went to a fancy dress in wax jacket, wellies and flat cap, carrying a fishing net and a copy of the Yellow Pages - F J Hartley.
One of those one-piece old fashioned underwear thingies, and with a felt pen draw squigly lines all over it like it is a jig-saw. If you're any good with drawing, draw a nice picture on it first.