There are plenty of synonyms for the phrase "sure thing", which means something that is guaranteed to happen or be successful. Some common synonyms include "certainty", "guarantee", "promise", "assurance", "certitude", "conviction", "positive outcome", "reliable", "indisputable", and "definite". Other synonyms may depend on the context, such as "slam dunk" in sports, "dead cert" in gambling, or "no-brainer" in decision-making. Whatever synonym you choose, it conveys the sense of absolute confidence and trust in the outcome or likelihood of an event or action.