WordNet records no synonym and no antonym for skateboard. It is the only member of each of its synsets, 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.
a board with wheels that is ridden in a standing or crouching position and propelled by foot
ride on a flat board with rollers attached to the bottom
Synonyms, antonyms and the broader and narrower relations for skateboard come from Princeton University’s WordNet. Full sources and licences.