Poland and Croatia

It doesn't make geographical sense, no. The two countries are not adjacent, do not share the same currency or language or culture.

But if you can find an easy way to fly between them - say a direct flight from a Polish city to a Croatian city - then go for it. Play around on Google Flights - plug in the main Croatian cities like Zagreb, Split, and Dubrovnik and try to find direct flights between say Krakow or Warsaw. Maybe you'l find flights only on certain days of the week.

Figure that flight link out first, then plan separate itineraries based on the flight connection. Fly into one country, out of another for your whole trip, as an Open Jaw booking.

You could probably find a long bus or train connection between countries - personally, I wouldn't do that, though.

If you want to hop quickly between countries, look for flights at www.skyscanner.com. Warsaw may have more options than Krakow.

By train, the fastest connections between Krakow and Zagreb take 17+ hours with 2-3 connections and travel via Czechia, Austria, and Slovenia, not Slovakia and Hungary. Looking Up Train Schedules and Routes Online gives you the Deutsche Bahn train schedule link and tips for using it.

Go a little further south to unexplored territory and enjoy the beaches of Montenegro (Budva and Ulcinj); then you might want to go up into the mountains or go a couple hours up the coast to Kotor and Dubrovnik.

Then hop a non-stop flight from Podgorica (Titograd) to Budapest.

From Budapest, after 3 or 4 days, there is a direct flight to Lviv (I suspect it will be then next "must see" destination to be ruined by too many tourist not too many years from now);

and from Lviv, after a couple of days to Krakow. All these are on discount airlines, direct, nonstop and are 1 to 1.5 hours in duration; and depending on how much luggage you have will average about $100 a flight.

from Krakow take the train to Warsaw and fly home.

I throw this out because I like Montenegro and am returning again to do the first half of what i suggest above in September and the second half in December and just happen to know the flights and the route.

No, it does not make geographical sense. If for some reason you want to visit these two countries go by air between them.

The fact is that ground connections between even contiguous countries in the Balkans can get hairy, though Zagreb-Ljubljana (Slovenia) and Dubrovnik-Kotor (Montenegro) work OK. So I see no harm in combining Poland and Croatia if you can find a good air connection between them. Do pay attention to which (if any) airlines have flights that operate daily or nearly so. I wouldn't want to take an airline that flew only one or two days a week, because if my flight got canceled...

I actually returned to the US (Washington-Dulles) from Zagreb in 2015. I started on Air Croatia and connected (I think) with a KLM non-stop in Amsterdam. It worked out fine. The Zagreb airport is small and a piece of cake to deal with.

I was just in the Zagreb airport 10 days ago. It has been upgraded and renovated. It is bigger, nicer, and more pleasant. Lines go quickly.

acraven is pretty much correct. There are two ways to explore the Balkans. You stay in a compact area and utilize local transportation which is generally slow or you just accept the fact that you are going to have to take a flight. We've been on flights our of Podgorica and its an easy airport. We used Air Serbia the last time and will be using Wizzair this time. Yes, at best there are two connections a week and I guess there is some gamble in that. Worst case scenario is you end up driving to another airport. We found ourselves in this position about 7 years ago in Bulgaria and survived. With these smaller airports located close to the cities they serve a 1 to 1.5 hour flight really is about 4.5 to 5 hours door to door; not bad in my world. Most every trip we take, and we do 3 most every year, we do some other Eastern/Central European city and Budapest. We pick places that have non-stop 1 - 2 hour flights on discount airlines on to Budapest. Once in 15 years we had to change the schedule.

What a coincidence, we are doing the same trip summer 2018. I am just starting to plan. I have the dates and places mapped out am now looking at hotels. Anyway, we are going to fly from Warsaw to Zagreb then rent a car. As of now, there are no flights from Kraków that we could find, only Warsaw. I believe the flight is only about 2 hours. When are you going? which cities in each country?