We will arrive into Zagreb and leave via Bucharest so those two points are stable. So far I have figured maybe:
3 days in Zagreb, one a trip to Plitvice
3-4 days in Budapest
1-2 days in Cluj
3 days in Brasov, mostly day trips to surrounding area castles
1 day in Bucharest
But that leaves a lot of time unused, obviously travel time needs to be accounted for but I'm trying to figure out if I should add more days to areas or more cities to stop in. We will be traveling in August.
I can tell for Romania - in your way from Cluj to Brasov maybe you can stop for one day in Sighișoara you will simply love it.
For Brasov you can use one day to see the city (go to Biserica NeagrăTâmpa,Strada Sforii, the center, the White and the Black Towers) if you like mountains trips you have Poiana Brasov (you have a bus that goes there from the center) or Piatra Craiului Mountains (both are one day trips - first one is easyer but the second is a bit more intense and can be even a 2 days trip and you can spend one night at Cabana Curmătura - Prăpăstiile Zărneștilor)
You can visit Râșnov Fortress (you have a train that goes there check http://www.regiotrans.ro/ - sorry is only in Romanian language. (If you want to visit Dracula's Castle there is no train so you have to check the buses).
In your way to Bucharest you can stop in Sinaia to visit Peleş Castle (from Brasov to Sinaia you can take the train and as well from Sinaia to Bucharest)
check this websites for trains: http://softrans.ro/mersul-trenurilor.html, http://www.regiotrans.ro/mersul--trenurilor--brasov--bucuresti--nord or http://www.mersultrenurilor.ro/ - they are 3 different companies so you will have different schedules
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