outdoors has 6 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. Its one recorded opposite is indoors.
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.
where the air is unconfined
“he wanted to get outdoors a little”
outside a building
“in summer we play outside”
Synonyms, antonyms and the broader and narrower relations for outdoors come from Princeton University’s WordNet. Full sources and licences.