activity has 7 distinct synonyms spread across 4 of its 6 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 inactivity.
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.
any specific behavior
“they avoided all recreational activity”
an organic process that takes place in the body
“respiratory activity”
(chemistry) the capacity of a substance to take part in a chemical reaction
“catalytic activity”
a process existing in or produced by nature (rather than by the intent of human beings)
“the action of natural forces”
the trait of being active; moving or acting rapidly and energetically
“the level of activity declines with age”
Synonyms, antonyms and the broader and narrower relations for activity come from Princeton University’s WordNet. Full sources and licences.