I just received the Million chart for Italy, which in fact includes many countries including Croatia. I was surprised to see that nearly all of Croatia is covered in a maze of Restricted zones which go from FL95 to the surface. So how on earth does anyone fly VFR there?
If you have looked closely, you will also have noticed that there is class D airspace all over the country above 1000 feet AGL anyway.
Hence, all cross-country flights are controlled flights anyway, where ATC will tell you where you can and can’t go. “Like IFR”, some people would say (although that is an inaccurate statement). You therefore don’t really have to worry about the restricted areas so much.
Also, the general rule is that the more and the bigger restricted areas there are in a given country, the less of them (will be active at the same time. Otherwise, as you said, one wouldn’t be able to fly in countries like Belgium, France, Croatia and Poland.
Croatia is very easy for flying GA. Lots of friendly airports and good, laid back ATC – essentially it is all CVFR flying.
Croatian ATC is as good as the best in Europe.
It is really wonderful, flying out of the UK, across France (“military active”, you fly along a narrow corridor only, and ELP is often poor), Switzerland (competent but strict, “military active”), Italy (struggles with routings, etc) and then you leave the Alps behind and speak to Pula and then everything changes; it just becomes relaxed and friendly.
The preferred routes shown on the VFR charts (ADRIA1 etc) are just for those who want to fly very low and look at the scenery from close-up.
So you call Zagreb just before you get to the border, tell them where you’re going, and just do what they tell you, relying on them to keep you out of trouble? I’m talking VFR here, though it becomes a lot like IFR except presumably you tell them if you need to avoid cloud?
Yes, they even have your VFR flightplan already. Just tune in with your current altitude and next waypoint directly upon initial call, and Croatian ATC will tell you something like “radar contact” and “proceed direct to”. That’s it. And otherwise, just request something like: “request sightseeing above city centre Zagreb” or “due clouds, request heading” or “request decent”, “request climb” etc. Croatian ATC is always very accomodating, so don’t be afraid to ask. 
