Last year together with other 5 single engine planes I flew from Italy to the Black Sea via Greece and Turkey. Return was through Romania, Bulgary, Hungary and Croatia.
Hungary was very easy and hassle free: we landed at Budapest International but I saw lots of grass airfields along the way.
In Bulgary we stopped at Sofia International and things were quite straight forward. You need to request prior entrance/overflight permit a few days before.
Romania was a bit of a burocratic nightmare: prior entrance/overflight permit a 3 days before each flight in the country. We stopped at Arad, Bucharest and Costantia, all big airports. This last airport does not have readliy available avgas (which is expensive in the whole country at 2 US$ per litre). VFR is not common and we were continually requested to report over IFR rps. Didn't notice any grass strips.
At Istanbul we landed at Sabiha Gorcka International which was just great.
The Danube was beautiful.
Ciao,
AE