Slovenia, Croatia, or Austria

Salzburg is delightful! You might want to add it to your Austria leg.

And/or Graz. Or the Alps. No shortage of options. There's also Bratislava, if Slovakia is open; I haven't checked.

Slovenia's also lovely. Ljubljana is a charming, not-too-large capital city. The time you have available would allow you also to see Bled and a bit of the Julian Alps. Maybe also one of the caves.

I agree that the Croatian coast is a major detour, but if you are more city people than countryside people, you could combine Ljubljana with Zagreb. Those two cities are connected by both train and bus. Zagreb's larger and has more museums. It lacks the river Ljubljana has, but Zagreb's bi-level historic district is very nice.

BUT in terms of logistics, it seems fairly unlikely that you'll be flying out of Budapest if you have to implement your back-up plan. If you can't enter the country as tourists and have to make a last-minute change to your itinerary, won't it be difficult to arrange a flight to Budapest to connect with your current ticket? I'd recommend flying in the night before since you'd have a transatlantic ticket at risk, but in this situation you wouldn't be able to leave the airport. I wonder whether it would be a good idea to call your airline and see what the procedure would be if you cannot get into Hungary a week before you're supposed to fly home from there.

In any case, there are few flights from Ljubljana. You will find more from Zagreb, but neither city has direct flights to Budapest. You'd spend extra hours making a connection (likely more time from Ljubljana) and costs look really high to me. You can check your precise travel date on skyscanner.com.

We loved Ljubljana and Lake Bled, as well as the Julian Alps. Slovenia is very easy—-everyone speaks English, and the tourist infrastructure is well-enveloped (although I don’t know how they fared the past year).

At Bled we stayed 3 nights at Penzion Kaps; I highly recommend it.

https://www.penzion-kaps.si/

Thank you! From what I can tell on Hungary is that we can enter to go to the airport as long as we show our reservations and go straight there. We will arrive in Europe on July 31 and won’t be in Hungary until August 9 so we are hoping to make the decision last minute while we are over there in case things change. As of now, we are stopping in Bratislava for a day (if we get to tour Budapest). If not, sounds like Slovenia may be our best option— looks like it’s gorgeous!
Thanks!

Slovenia or more Austria would be equally nice choices.

I am with acraven on the risk of flying home from Budapest if you can’t tour, though. I haven’t checked it myself, but entering and going straight to the airport doesn’t mean you can spend the night in a hotel there the night before your flight, does it? It looks like you have the choice of risking not being able to dented /stay or not making your flight by being delayed - or just changing your return flight and feeling more secure (plus not wasting a day just to get to the airport).

I am traveling quite a bit this year, but am also trying to minimize my stress and the risks for the things that ARE in my control.

Another strong recommendation for Slovenia. I spent 2 weeks there in September 2018, and was blown away by its beauty. Reminded me a lot of New Zealand in pure natural beauty. I rented a car and my favorite spots were Lake Bohijn and Logar Valley. Also very inexpensive.

In Vienna I would add a day and take a day trip to Bratislava (1h by train). Rick Steves’ guidebook will tell you what to do once there. I would also include going to Salzburg, Munich and Bavaria since you’ll be nearby if going to Hallstatt.
I see a lot of people mentioned Slovenia which is wonderful, but I would combine that with Croatia. You may want to do this another time since it’s further away.

Here's another vote for the Slovenian combo of Bled and Ljubljana. You should also look at throwing in a bit of Piran while you're there.

Thank you all so much for this info!! I think we will do Slovenia as our backup plan (and, maybe, a whole trip in the future).
For the airport part, from what I am reading, we are allowed to go to the airport. We are hoping to keep that flight because of price as well as hoping Hungary will open by the time we arrive. Am I not interpreting this right? All the COVID entry stuff is so unclear and confusing.

“ Rules for persons transiting Hungary
A non-Hungarian citizen arriving from abroad may enter the territory of Hungary for the purpose of transit, if he/she is subjected to a medical examination at the time of entry and the medical examination does not establish the suspicion of infection.
An additional condition for entry is that the non-Hungarian citizen coming from abroad
a) have the entry conditions provided for in the Schengen Borders Code,
b) satisfactorily proves the purpose of the journey and the country of destination of the journey; and
c) the entry into the country of destination and, to this end, entry into a state neighbouring Hungary on the route of the planned journey is ensured.
A non-Hungarian citizen arriving from abroad may travel on the specified route only during the transit through the territory of Hungary, and may stop at the specified resting places only for reasons strictly necessary for the transit, such as health or technical reasons, and at the specified time - but not exceeding 24 hours - is obliged to leave the territory of Hungary.
An admitted non-Hungarian citizen arriving from abroad may interrupt and stop his/her travel on the specified route in addition to stopping at a specified resting place in a technical or medical emergency situation requiring immediate intervention.

Special rules on transiting to or from an airport
Transporting of the transiting person to or from an airport can take place under general rules for transiting.
Such transporting may take place on the one hand within the sphere of business or economic activity of the road passenger transport operator, in which case the driver employed by such an operator is subject to Article 9/A and 9/B of Government Decree No. 408 of 2020 on travel restrictions during the period of disease control preparedness (hereafter: Government Decree). In pursuance of these Articles, the driver can enter Hungary without restrictions, and regulations appearing in Article 14 of the Government Decree apply to persons entering for transit purposes.
On the other hand, general rules on transiting shall apply to such persons too that use their own vehicle when travelling to or from an airport.
During travelling to or from an airport, transiting persons and the passenger transport operators transporting them are obliged to use the shortest route between the airport and the designated transit route.
Entry or transporting in any other case than those listed above can only take place under the general rules on entry (Articles 2-7 of the Government Decree).”
http://www.police.hu/en/content/information-on-general-rules-of-border-crossing

So many great choices! We did Slovenia and Croatia in 2018, which included Venice prior to our tour. Combining both made it a wonderful adventure. We loved Slovenia, especially Ljubljana. Have to say it was my favorite city on the tour. In 2019 we did a self guided tour of Budapest, Austria and the Czech Republic. We stopped in Cesky Krumlov as well. All the best as you plan your journey.