extend has 25 distinct synonyms spread across 15 of its 17 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.
extend one's limbs or muscles, or the entire body
“Stretch your legs!”
extend or stretch out to a greater or the full length
“Unfold the newspaper”
open or straighten out; unbend
“Can we extend the legs of this dining table?”
use to the utmost; exert vigorously or to full capacity
“He really extended himself when he climbed Kilimanjaro”
prolong the time allowed for payment of
“extend the loan”
Synonyms, antonyms and the broader and narrower relations for extend come from Princeton University’s WordNet. Full sources and licences.