Dubrovnik is close enough to Kotor that it's a realistic day trip — about 75km, roughly 1.5 to 2 hours depending on traffic and the border queue. But it does mean crossing an actual international border between Montenegro and Croatia, which trips people up more than it should, mostly because they didn't expect a border crossing to be part of a "local" day trip.
What documents do you need
A valid passport is the safe answer for anyone who isn't an EU or regional citizen — bring the physical document, not a photo of it. EU citizens can typically cross with a national ID card. If you're driving a rental car or scooter, you'll also want to carry the vehicle registration and rental agreement, since border police occasionally ask for it, and definitely confirm with your rental agency beforehand that cross-border travel is covered by the insurance — this is the detail most people forget, and it matters far more than the passport question.
If you're on someone else's booked transfer or organized tour, the documentation is usually handled by the driver or agency and you just need your own passport in hand.
How long does the crossing actually take
This is the genuinely unpredictable part. In the shoulder season (spring, early autumn) the Debeli Brijeg crossing can take as little as 10–15 minutes. In peak summer, especially July and August mid-morning to early afternoon, queues of 45 minutes to well over an hour are common, particularly for cars — pedestrians and those on organized coach tours sometimes have separate, faster lanes. If you're driving yourself, leaving early in the morning (before 9am) or crossing in the evening dramatically cuts your wait.
There is no way to "skip the line" legitimately, and Montenegro-Croatia border staff are not swayed by being in a hurry. Build the uncertainty into your schedule rather than fighting it.
Driving yourself vs. organized transfer
Driving gives you control over timing and lets you stop at Cavtat, a much quieter coastal town just past the border, on the way. It also means dealing with Dubrovnik's notoriously difficult and expensive parking directly, and being the one responsible if the border queue runs long and eats into your day.
A private transfer or organized day trip removes both problems — the driver knows current wait times, alternate routing, and where to park (or drops you at the gate and picks you up later), and the fixed price is agreed before you leave, so a long queue doesn't cost you anything extra. For a first visit, or if your schedule for the day is tight, this is usually the less stressful option.
What to actually do once you're in Dubrovnik
The Old Town is the obvious draw — walk the City Walls early if you want to avoid both the crowds and the heat, since there's very little shade on the walls themselves. Cable car up to Mount Srđ gives a view over the walled city and the Adriatic that's worth the ticket price, especially late afternoon. If Game of Thrones filming locations interest you, several walking tours specifically cover them, starting near the Pile Gate.
Given the round trip and border timing, a single day trip realistically gives you 4–6 hours inside Dubrovnik itself once travel time on both ends is accounted for — enough for the Old Town and City Walls comfortably, tight if you're trying to add the cable car and a proper sit-down lunch as well.
Currency and practical notes
Croatia uses the euro, same as Montenegro uses informally (Montenegro isn't in the EU but uses the euro unilaterally), so there's no currency exchange to worry about crossing the border. Mobile data roaming between the two countries can behave inconsistently depending on your carrier — worth checking before you go if you're relying on maps or messaging apps during the day.