routine has 14 distinct synonyms spread across 4 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.
an unvarying or habitual method or procedure
a short performance that is part of a longer program
“he did his act three times every evening”
a set sequence of steps, part of larger computer program
found in the ordinary course of events
“a placid everyday scene”
Synonyms, antonyms and the broader and narrower relations for routine come from Princeton University’s WordNet. Full sources and licences.