pensive has 9 distinct synonyms spread across 2 of its 2 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.
deeply or seriously thoughtful
“Byron lives on not only in his poetry, but also in his creation of the `Byronic hero' - the persona of a brooding melancholy young man”
showing pensive sadness
“the sensitive and wistful response of a poet to the gentler phases of beauty”
Synonyms, antonyms and the broader and narrower relations for pensive come from Princeton University’s WordNet. Full sources and licences.