Croatia is the, next to Slovenia, the easiest country of the former Yugoslav states, to ride.
Pretty much everybody speaks English, there’s a lot of tourism (particular along the Adriatic Sea), and it’s an EU country.
It goes south along the Adriatic Sea, interrupted by Bosnia for some kilometers, to continue to Dubrovnik, which is also Croatian. It also stretches all the way to Serbia in the East. If you want a touch of Balkan, but stay in EU, you can swing down into Croatia, make you way up again, East, then just before Serbia, ride into Hungary for a short while, just to enter Romania after a 180 km or so.
Yes. Croatia is part of EU, but not Schengen.
Green insurance card: No
Technically not needed, being an EU member country. And they will not ask for it. But get it anyway.
Vehicle Registration Certificate: Yes
Yes, always. The original.