Hello. You did not present the sentence to which this question refers, which makes it impossible for it to be answered. However, I will try to help you in the best possible way.
In order for the meaning of the sentence to be changed if the word "activity" is replaced, you should use words that are not synonymous with "activity". Synonyms are words with different spellings, but they keep the same meaning. In this case, it is only possible to modify the meaning of the sentence, replacing "activity" with antonyms such as "inactivity," "inactiveness" and "inaction." Antonyms are words that have completely different meanings.