fly into Croatia and drive into Hungary/Poland

Have you checked the drop off fees? They can be quite large for dropping a car of in a country different from the one you rented it in. They will probably cover any air fare and then some.

You are aware that returning a rental car in a different country than where you picked it up can incur a hefty drop off fee; I’ve heard as high as $900?

Definitely check the fee to rent in one country and drop off in another -- which, yes, can be stunningly high. But not always. Sometimes it may be "reasonable" (I define reasonable = not so high that you can justify it, if it allows you to do an itinerary that you could not do otherwise without crazy hoop-jumping). I've seen drop-off fees over €800 (I spent more than one day crossing borders to avoid those), and as low as $100 (I've accepted that as the cost of doing an itinerary I wanted). If you find a too-high one-way drop-off fee, you might shop around a bit, maybe you will get lucky with another agency. But do not assume it will be a no- or low-cost option (it frequently is not). The fact that you're talking about Eastern/Central Europe is another wild card in the mix.

I would check the cost of your car rental plan before you get any further with your planning. COVID and border-crossing details would be less of a concern, at least in terms of nailing down the "big picture" part of planning.

There are trains and buses that go from Budapest to Zagreb. These are a much better approach than renting a car for a one-way trip.

Last year, before the airport opened to US citizens, you could cross the border from Croatia with no COVID documents. So I booked a flight to Zagreb and had plans to have a transfer service pick me up at the airport. Okay, sort of expensive but my time on the ground is valuable. The cost I believe was to be $300. Then the airport opened and I changed my plans. But if anyone is interested, the transfer service is a company I have used for maybe 10 years now. Good folks.

The Croatian / Hungarian border is open, no documents required other than a passport, and reportedly the waits are not long. Enjoy the trip.

Also, I believe I read that croatia dropped the proof of lodgings recently. Double check. We went from Croatia into Bosnia. We held up our passports and they just waved us through, didn’t even check them. Don’t know about Hungary, restrictions and requirements are changing, it seems, weekly.

Hungary's last change was August 8, 2021. So not everyone is changing daily. If Croatia did drop the hotel proof requirements (which by the way you dont need to transit the country -- but best to have your reservation in Hungary in hand to demonstrate you are in transit) then it would be first change since late Spring last year (and to the better). As far as I know, Bosnia and Montenegro have remained constant since late Spring; as have a number of other countries. Western Europe has been bouncing around a lot. That's why parts of Eastern Europe might settle better for those with a lower risk threshold.

I will just answer the last part of your question, I went to Auschwitz in August. It was still very busy but I was able to book a tour just a day and a half beforehand. I have heard that they usually sell out weeks in advance but that has changed with the pandemic. Might even be easier for you in the off season.

When I traveled from Montenegro to Croatia in September via the land boarder, they didn't even ask about my accommodations. I don't know anyone who had to show proof so I don't think it was being enforced very strictly.

Thanks all for all of the great responses. We plan to fly into and out of Croatia so the car rental will be a round trip rental. My son is studying in Italy during the summer so he is trying to limit his time spent in Schengen areas.

Since holocaust history seems to interest you, let me know when you will be passing through Budapest and I can send you some sort of out of the way ideas on the subject.

James E: If you have a bunch of WWII stuff about Budapest, could you post that? My great uncle actually worked for the Nazis in Budapest until 1944, and then the family (of germanic ethnicity) fled to Germany, so I have connections to this stuff. As to what he did, I don't know, and I won't ask his son who is still alive.