Most set-up is an adjective used to describe a situation where something is arranged or planned carefully in advance. It implies a great deal of preparation and organization. The antonyms for this term would be casual, unplanned, or spontaneous. A casual experience or event is one that occurs without much planning or forethought, such as a relaxed get-together with friends. An unplanned event is one that is not organized beforehand, such as an unexpected visit from a friend. A spontaneous event is one that happens in the moment, without premeditation or planning, such as a road trip or a sudden decision to go out for a meal.