Itinerary help for Balkans

That looks like a great trip! It's a little bit busy but given that you're in your 20s, I'm sure this pace will work out. I would only say that you may have one night too many in Dubrovnik, but you can always do a variety of day tours. Lokrum is quite small - I spent a couple of hours there so it definitely isn't the same scale as, say, Korcula. Did you check whether you can get back and forth from Mljet in one day based on the ferry schedules?

That will be a very interesting trip.

Have you explored the possibility of renting the car in Kotor but dropping it in Podgorica? It's not that the bus is a problem, but not having to be in Kotor for the bus departure at a particular time might allow you to squeeze in a visit to another part of Montenegro. It's possible that there might not be a drop fee.

I liked Ulcinj, Herceg Novi, Cetinje, and Budva. All have old historic quarters that are not terribly large, so you could hit more than one per day. Of the four, Budva most reminded me of Kotor, and it was getting quiet by early October.

There are some lovely trips through the mountains that I did not take because I was dependent on public transportation.

I imagine you already know that Podgorica is not a highlight.

The Bay of Kotor is not to be missed, but you might get a good enough view of it from your inbound bus. I just don't remember.

Check online for cruise ships expected in port (Kotor) while you're in Montenegro. Consider reallocating your Kotor/outside-Kotor time if that would allow you to avoid huge cruise contingents.

Border-crossing buses usually don't run very often. I recommend buying tickets for those as soon as you reach the departure city.