I've been to Croatia a number of times (and highly recommend it for both flying and a holiday location) but have never been to Serbia.
Austria has just brought in a strange king of aviation tax, which you can read about
here. But instead of collecting it via landing fees etc, you have to nominate a tax representative. This effectively curtails GA flights to Austria... Sub 2000kg aircraft are exempted.
There is still "trouble" in some parts of the old Yugoslavia (south of Serbia, I think) and my insurance covers overflight there but not landings.
Re guided flying tours, there is the occassional group outing though they tend to go further south, into Africa. Down there, once out of Europe, things get considerably more complicated logistically.
This is a recent account from a pilot I know of a trip down to Egypt and then Sudan, the latter bit having been abandoned due to permit problems. There are some seriously hard-core pilots in Europe, often flying long-range Mooneys, who have done amazing trips. But there is no getting away from the fact that flying is flying, whereas the logistics of getting avgas and overflight/landing permits just get more complicated... which is why I don't bother.
But those are not guided tours for pilots unfamiliar with Europe, as far as I have seen. Basically everybody flies on their own. In some cases, somebody organises the hotels and fuel etc, for a fixed up front fee, which is rarely trivial. Personally, I have not participated in such trips because I am not retired
and I like to make my own weather decisions, without any pressure from a group which one needs to keep up with.
Flying in Europe is not a problem, especially if you have an IR and have an aircraft whose registration matches the IR issue country, so you can fly IFR (there are not many N-regs for hire in Europe, and there is no way to validate an FAA IR into a Euro IR, even temporarily). You then use Jepp plates, and apart from stuff generally unfamiliar to Americans such as PPR, PNR, Customs PNR, and limited airport opening hours, avgas and Customs availability, the flying is the same. You use different internet sources for weather, and you file the ICAO flight plan using services different to those used in the USA.
If you are flying VFR then you need to sort out a load of VFR charts, and plan the routes carefully w.r.t. controlled airspace, military airspace, etc. Much of the controlled airspace you can get clearances to enter, but not all. Obviously a good large GPS is a must.
A lot of the scenery in Europe is spectacular, and I have covered only a few bits of it in my writeups.
Europe, in its constant fight with the USA, is now proposing to force European resident pilots to carry European papers, in addition to the papers normally required by the State of Registry. This is even though the Euro papers may be invalid (non-compliant with FAR 61.3) for the US-reg aircraft being operated. So, as an insurance policy in case the idiots in Brussels do manage to pull this off, I am working through the Euro IR and its seven exams which are about 95% garbage
If those people in the USA who are buying Airbus and other hardware, especially military hardware, knew what the European regulators are doing to their own US-licensed pilots, they would not touch the stuff with a bargepole.