itinerary suggestions starting in Bulgaria or Hungary

Out of the countries you named I am most familiar with the Czech Republic. If you want to see just big cities don't bother with a car. Go by train. For countryside, small towns and villages car could be useful. As a good and not overwhelming source of information I consider Rick Steves book Prague and the Czech Republic. If I know how much time you have for C.R. and what your interests are I could be more specific with advising what to see and what is not in R.S. book.

I'll be traveling in that area this fall, flying into Belgrade from Riga and flying home from Budapest. After Serbia I'll join the RS tour of Bulgaria, then I plan to go overland through Macedonia, Albania, Montenegro, and Bosnia, probably finishing by revisiting Croatia. All these countries look really interesting. If you want to go north instead you should look into flying - buses and trains from Sofia through Serbia are very slow.

Last year I went from Budapest to Vienna to Prague by train. Had I more time, I would have added Brno for sure. I would have liked to see Krakow too, but it was too far on my short trip (12 days on the ground). As it was, I could have spent at least 2 days more in each of the 3 cities and still not seen everything. The trams and subways made getting around the cities easy, and many of the highlights are within easy walking distance of each other. And of course, there are lots and lots of places to sit, have a snack or a meal, and soak up the view and the atmosphere. There are also convenient day trips from each of the 3 cities, but I didn't have the time, sigh. Next year, maybe. . . .