Main answer: A simple and honest reference to the general nature or purpose of the trip suffices. Little or no elaboration necessary. If more than the listed destination country was visited during the trip, all such countries should be listed in the reason box.
Keep it simple. Short. Be truthful. For short trips, merely stating "holiday" works just fine . . . so long as that is the truthful answer. (Business trips should be described as a "business trip." No need to elaborate about the business purpose. No advanced degrees in sociology and government necessary to figure this out.)
Beyond that:
Context matters. Day trips hardly need much of an explanation at all. In fact, just putting "day trip" should readily suffice. Just "shopping" should suffice. Or "sightseeing." Of course any description should be truthful.
Brief trips likewise need little explanation. One or two word descriptions should easily suffice. "Work." "Holiday." "Business." "Vacation." "Visiting friends." "Visiting family." Again, it needs to be truthful.
Where what the applicant enters into the reason box is more important is:
-- listing countries visited during the trip in addition to the named destination (applicants should list every country visited)
-- a purpose or reason consistent with the duration of longer periods abroad in context with the applicant's life
Historically scores of applicants simply stated "holiday" even when they were abroad for several or many weeks, even months, even though a "holiday" of that length was incongruous with the other facts and circumstances in the applicant's life. Note what may make sense for a University professor or school teacher referring to a ten week holiday abroad in a time between ordinarily scheduled classes, versus what does NOT make sense, not so much anyway, if an individual who is employed in retail or manufacturing reports a three month holiday. Not many manufacturing or retail jobs easily facilitate three month holidays. And particularly not in back to back years.
Like a lot of the peripheral information provided attendant the application,
in the routine case these details are of very little if any import. IRCC personnel will, of course, readily recognize obvious incongruities (like the applicant working in manufacturing taking three month holidays abroad in back to back years), and depending on other information that might trigger additional scrutiny and inquiry. In contrast, if and when IRCC identifies a reason to have concerns or be suspicious about the applicant's accounting of travel and time abroad, that is when the details in the reason-box might be examined more closely, looking for clues (such as for indications the applicant may have been employed abroad and has not disclosed that employment in the work/activity history information).
THIS INFORMATION (the reason for the trip) HAS
NO DIRECT BEARING ON QUALIFICATIONS FOR CITIZENSHIP. It is peripheral information which IRCC is entitled to ask for to facilitate its assessment of the applicant and the other information which does have direct bearing on qualifications for citizenship.
Thus, the ONLY wrong answer is an untruthful or evasive or misleading answer.
