When it comes to finding the perfect synonym for the phrase "lightweight affair," there are a number of options available. Some possible alternatives include "casual fling," "brief dalliance," "short-term romance," or "fleeting liaison." Other options might include "passing infatuation," "temporary attraction," or "brief encounter." Ultimately, the best synonym for "lightweight affair" will depend on the context in which the phrase is being used, as well as the specific connotations and associations that the speaker or writer wants to convey. Regardless of the specific synonym chosen, however, the basic idea behind a "lightweight affair" is always the same: a fleeting, temporary, and relatively unimportant relationship.