The phrase "does on the sly" is often associated with secretive or deceptive behavior, so antonyms for this expression would include words like open, honest, aboveboard, and transparent. Someone who does things openly and honestly would be trustworthy and dependable, whereas someone who does things on the sly would be viewed with suspicion and mistrust. Synonyms for doing things on the sly might include words like sneaky, furtive, or covert, while antonyms might include words like forthright, candid, or upfront. Ultimately, the choice of words to describe behavior will depend on the context and circumstances surrounding the situation.