fun has 4 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
activities that are enjoyable or amusing
“I do it for the fun of it”
violent and excited activity
“she asked for money and then the fun began”
a disposition to find (or make) causes for amusement
“her playfulness surprised me”
Synonyms, antonyms and the broader and narrower relations for fun come from Princeton University’s WordNet. Full sources and licences.