suggestions for travel from Dubrovnik to Belgrade - we have 3-4 days

Border-crossing trains and buses are extremely limited in the Balkans. You'll need to check locally to get information every step of the way. The information on the internet may be incomplete or wrong.

The obvious stops between Dubrovnik and Belgrade are Mostar and Sarajevo. I haven't going looking for bus service along that route, but I believe it exists.

Bucharest would not be my first (or second or third) choice of a place to visit in Romania, though if you're flying to Istanbul it may be the right place for you. Have you worked out how you're getting from Belgrade to Bucharest? Are you flying?

If you have time for a side-trip while in Belgrade, I can recommend Novi Sad. It has an historic center with a sort of hippie vibe.

*Thanks for your response and comments about crossing borders. We've been hearing some good things about Bucharest which is why we chose it, but that is flexible. We are planning as we go and love the flexibility to make adjustments. As I mentioned in the first post, our schengen clock and the RS Tour of Turkey are the only two pieces that are fixed in our itenarary.
Instead of going to Bucharest, we could travel through Bulgaria (sofia and Plodvid) We are trying to find a logical path to eventually get to Instanbul, to stay in non-schengen countries and find logical transportation between cities/countries. Our Schengen clock starts over in early March. Belgrade is on our list because we have a personal connection there.
We prefer to spend no less than 4 days and up to two weeks in a location so we can explore on a more leisurely basis and still have time for relaxed days.

Transportation between cities is a consideration on where we go. My initial question came from researching transportation between Dubrovinik and Belgrade. We have 4 days that are "open" between these two cities. We did read mixed reviews about a scenic train route between Bar in Montenegro and Belgrade. But, that's a 10 hour train ride.....so I'm not convinced. However, a flight to Belgrade from southern Croatia is fairly expensive.

It's nice to have options - but can be daunting as well.

I haven't taken the Bar-Belgrade train.

Places I liked in Bulgaria more than Sofia: Plovdiv and Veliko Tarnovo.

Places I liked in Romania better than Bucharest: Timisoara, Brasov, Sibiu, Sighisoara, Cluj-Napoca, Maramures area (needs car or tour), Bukovina (ditto).

It's not that Sofia and Bucharest have no points of interest at all; I just liked the other places more. In the case of Bucharest, you have to work hard to find attractive architecture because Ceausescu pulled down so many of the historic buildings.

When I wanted to get from Romania to Bulgaria in 2015, it seemed I pretty much had to travel from Bucharest. Then when I wanted to get to from Bulgaria to Montenegro, I found the best (bus) route began in Sofia.

We've been to Croatia 6 times since 2011. There are roughly 3 section - the coast, the area around Zagreb, and Slavonia, to the east of Zagreb.

It would be helpful if you put down your itinerary for Croatia.

We've been to Dubrovnik, Split, Trogir, Sibenik, and Zadar. All are a little different, but variations on a theme. The best are D and Split.

Dubrovnik - 2 nights - take ferry to Split
Split - 3 nights - take day trip to Trogir
Rovinj - not been there
Plitvice Lakes - arrive afternoon - stay overnight in park hotel and do the morning - take bus to Zagreb
Zagreb - 2 nights
Train to Beograd (not Belgrade, we are not British) - stay in the Hotel Moskva - $137/night - best hotel in town

You may also want to do Novi Sad in Serbia

I don't know what your family/personal connection is, but there's lots of Serbia we haven't seen. We did visit family villages in the Vojvodinja using a TI guide. PM me for the name.

Trains out of Beograd are limited, for some reason. There's a train to Sofia, Bulgaria. If you want to go to Bucharest (which we quite liked), look at rome2rio - bus may be the best bet.

Skyscanner shows a direct flight from Dubrovnik to Belgrade for $75 or you could take a bus to Mostar and a train to Saravejo and fly nonstop to Belgrade for $90. You have a fabulous adventure planned.

I would do the 10 hour train ride between Bar and Beograd. It sounds amazing.

https://www.lonelyplanet.com/articles/belgrade-bar-railway-the-ultimate-balkans-train-trip

My limit on trains is maybe 4 hours. I have flown Titograd to Belgrade so thats an option. That way you get to see a bit of Montenegro which I really enjoyed. I also once drove from Sofia to Istanbul. I say "I drove" more correctly someone else drove. We made a 5 day trip out of it. That was the second trip to Bulgaria. Hope to make it back again some day. Beautiful country and the fishing was pretty fair as well.

It's more than possible to go from Dubrovnik to Mostar to Sarajevo to Beograd by bus. But I cannot tell if you are traveling now or in the future. If you are traveling now the buses are likely not running at this time of year. We found in 2014 that the bus from Mostar to Dubrovnik was done for the season on Nov 1. We had to get a private driver.

It's really helpful to include the time of year in your original posting.

I hope you enjoyed Balkans and Belgrade especially, Istanbul I really enjoyed last November, How was the rest of Turkey? Any recommendations?

Danilo

OP's last post was in mid-February. That's a long time ago in pandemic-years. A lot has changed since then.

I don't know which Rick Steves Turkey tour they were planning to hook up with, but it's a pretty safe bet that tour ain't happening, and the OP has probably had much more urgent matters to focus on. Their trip through the Balkans and onward to Turkey has surely been stopped in its tracks, and probably derailed altogether, due to nearly every country including Turkey closing its borders (unless they made it to Turkey before the border closed a few weeks ago). So it's probably pointless to continue responding to this thread with suggestions for itineraries.

Hopefully they made it back home before everything clamped down, or at least are hunkered down someplace good-enough, where they will be OK for a few months riding this out.