Belgrade and/or Bucharest

You don't say how much time you have for the three cities already on your itinerary. If you add either Belgrade or Bucharest, would you be subtracting days from the other cities or adding days to your overall trip?

I've been to Bucharest but not Belgrade. I did like Bucharest. If you go, I'd recommend staying three nights so you have two full days.

But depending on how much time you have, adding days in your three other cities might be a better choice.

Ive bern to both. Hands down Bucharest. I have traveled all over Eastern Europe and Belgrade was the least comfortable for me.

The question of are you spending enough time in the other cities is legitimate. We are all different but I would say 3 nights in Prague, 3 to 5 in Vienna and 4 or 5 in Budapest as absolute minimums. Then add a day to the first arrival city.

Having first done that, if Budapest is your last stop I would hop a discount flight to the place that most interests you. The Budapest airport is simple and the flights are cheap. To some degree it depends on how much time you have but what comes to mind are

Malta
Bucharest
Montenegro
Albania
Sofia
Krakow
Lviv or Odesa but those require some special, but doable, travel arrangements.

Are you specifically looking for a larger city? Otherwise, I might suggest Brasov, which is really a charming city not that far from Bucharest (I think it's about a 2 1/2 hour drive). There are a number of day trips you could do while you're there (Peles Castle, Bram Castle and the Bear Sanctuary), and the city itself is just really lovely. It was one of my favorite places in Romania.

Wow! Wonderful comments. I tend not to give enough info at first so I will elaborate a bit. Our plan is to start in Prague, possibly 4-5 days since jet lag will be an issue coming from the States; on to Krakow (3 nights?); then Budapest (4-5 nights?).

If we move on from there, we won't be deducting any time from the first spots. It would be new additions since we're retired and very flexible. We've already done Vienna several times so we aren't interested in going back there.

No, we aren't necessarily looking for only bigger cities. In fact, that's also why I asked about day trips or other possibilities. I've been reading about Brasov and Sibiu and we would be very open to some smaller areas, even around Prague, Krakow and Budapest. RS doesn't have much on Romania and Serbia, which is why I'm trying to do more research and also reach out to all of you.

Thanks!