scheme has 8 distinct synonyms spread across 6 of its 7 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 elaborate and systematic plan of action
a group of independent but interrelated elements comprising a unified whole
“a vast system of production and distribution and consumption keep the country going”
an internal representation of the world; an organization of concepts and actions that can be revised by new information about the world
Synonyms, antonyms and the broader and narrower relations for scheme come from Princeton University’s WordNet. Full sources and licences.