WordNet records no synonym and no antonym for sandwich board. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
signboard consisting of two hinged boards that hang front and back from the shoulders of a walker and are used to display advertisements
Synonyms, antonyms and the broader and narrower relations for sandwich board come from Princeton University’s WordNet. Full sources and licences.