"Pop in" is a casual way of describing a sudden and brief visit. Some synonyms for this phrase include "drop in," "swing by," "visit briefly," "stop over," "call in," and "make an appearance." Each of these variations delivers the same meaning; however, they may have different connotations based on how they are used in context. "Dropping in" might suggest an unannounced visit, while "stopping over" implies a quick visit on the way to another location. "Swinging by" is informal and suggests a relaxed, unplanned visit, while "making an appearance" has a more formal tone. Ultimately, the synonym chosen will depend on the tone and context desired.