picnic has 10 distinct synonyms spread across 2 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.
any undertaking that is easy to do
“marketing this product will be no picnic”
eat alfresco, in the open air
“We picnicked near the lake on this gorgeous Sunday”
Synonyms, antonyms and the broader and narrower relations for picnic come from Princeton University’s WordNet. Full sources and licences.