Croatia/Slovenia/Hungary

I would first play around on Google Flights and look at your multi-city options (I'm on east coast, and flying from west coast is a whole different ballgame). Do you have time to break up the trip (say, fly to London, fly out the next day, or do you intend to do it in one go, even if booked on separate flights?)?
You could rent a car for northern Croatia (best to have one for Istria) and Slovenia, do a loop, turn it in and train or bus to Budapest.

Budapest is a needless addition in my eyes. It’s not convenient in any sense, and you’re barely spending a day or two in any destination as it is. Put those days back into Croatia/Slovenia (Ljubljana is an easy call imo.)

You’re not going to find anything non-stop here, it’s just too far and the airports are smaller. Tickets will not be cheap, so look at it from the standpoint of where you would ideally fly in and out of. This would probably be into Dubrovnik and out of Ljubljana or Zagreb. When I was searching for airfare, I found the typical aggregator sites had the worst and/or most limited service available. Dealing directly with the airline, in my case United, I found a whole slough of flights available that were not even listed via the big websites. And the price was about the same. I found pretty much the ideal itinerary through them. So, check direct with airlines and punch in your destinations, don’t try to piecemeal these together yourself.

Budapest really deserves more than 2 or 3 days. I'd hold off on it until you have more time.

Montenegro is a lovely country, though small. The national parks (not accessible by public transportation) are said to be very nice; I couldn't get to them because I had no car. The issue with having a rental car for the jaunt to Montenegro is that the driver will have to keep his or her eyes on the road all the way around the magnificent Bay of Kotor. I was very happy to sit back and look out the window of a bus.

I flew home from Zagreb back in 2015 with a single stop in Europe (maybe Munich). It helps that my home airport is Washington-Dulles, which has quite a few non-stops from Europe.

Zagreb and Ljubljana are 2 to 3 hours apart by either train or bus. I think bus service may be more frequent.

I think the amount of time you have allocated for each place seems reasonable, but what's missing
is the transport schedule and timing, which can eat substantially into the time allocation you have.

Arriving in Split on Tuesday, for example, and leaving on Friday, gives you two full days, but it's 4
days on the calendar.

Seeing Plitvice as a day trip is not always recommended either, due to the # of tour buses that show
up in the middle of the morning.

I don't think there is a train from Dubrovnik to anywhere. That is, Dubrovnik has no train station. You
either fly/bus/drive in/out.

So, my input is you have to dive a bit deeper into the transportation arrangements to be sure you know
what can fit into the amount of time you have. As for best way to/from SFO, again, you have to play with
things - Google Flights can help a lot here. If you want to minimize the # of connections/hassle, then you
are most likely sticking with one airline/alliance and that helps narrow down the options to the major hub
cities for that alliance. On the other hand, if you find a bargain flying to/from Paris, for instance, then you
might want to look into the bargain airlines in Europe (there are a lot of them) to see if they can get you
between the hub and your final destination. SFO obviously points you in the United/Lufthansa direction.

Renting a car is something you may need to do to get around Croatia and Slovenia. Public transportation
is possible, but you will lose time and potentially get lost trying to find bus stations, etc. Good news is that
it's not really an issue, other than gas costing around $8 a gallon. Buses are good between major cities,
(Ljubljana-Zagreb and Split-Dubrovnik), but less so in Bled, Plitvice, Julian Alps, etc. Flixbus is reliable
and provides very reasonably priced fares between a lot of places.

Also, rental cars rented in one country and dropped off in another incur non-trivial dropoff fees. Setting
the trip up where, for example, you finish the Croatian segment in Zagreb, drop off the car, take the bus
to Ljubjiana, and rent another car there mitigates that issue. Also, there is train service between Ljubljana
and northern Croatia.

I would also recommend saving Budapest for another time.

Hello to all who responded with your suggestions...truly appreciate it! LOL, when I look at the map the countries looks so close and doable for getting to/fro. Of course the "devils in the details"...so based on your feedback we'll save Budapest for another trip. I've been using Kayak to check on flights but just now got on to Google Flight and can see that it's laid out more clearly with more options so I will pursue that angle. Yes, I will definitely spend more time dialing in on our mode of transportation & associated time allocations for getting around. Back to the drawing board and thanks again for your input! I'll likely be back with more questions :).

Good choice--you will find so much to keep you within Croatia and neighboring countries.
My first impulse would be to do the kayak trip first, with maybe just a day or a few days before. But it really depends on how the flights work out. I think mid September will be ideal weather for that part, which is the most season-dependent. And ending in Istria in October during truffle season would be really nice, if you are in to that kind of thing!

If you had saved your trip to Kotor for the last day and continued a bit further to Podgorica, there are direct flights to Budapest on Tuesday and Saturday for about $100 (90 minutes). It's how I would get home from Croatia.

Actually after seeing Dubrovnil, Kotor isn't that special, but Perast and Budva can be interesting.

Based on the great suggestions provided I've revised my itinerary. 1) nixed Budapest, 2) pushed out Sea Kayaking trip 1 week so we can attend Truffle festival, 3) researched ground transport/timing.

9/21-10/16/24 revised itinerary:
- depart SFO/LHR (maybe overnight)
- Dubrovnik - 3 nights
- Elaphiti Islands - 7 nights (organized Sea Kayaking adventure)
- day trip to Montenegro
- rent car in Dubrovnik - drive up coast to Split or Zadar (trying to decide which to sleep at)
- Split or Zadar - 2 nights: drive to Plitvice Lakes, other sights
- Istria - 3 nights (maybe Rovinj but open too other north spots on peninsula)
- bus from Istria to Trieste; train from Trieste to Ljubljana
- Ljubljana - 3 nights
- train or bus to Lake Bled
- Lake Bled - 3 nights
- train from Lake Bled to Munich or Frankfurt
- 2 nights either Munich or Frankfurt
- return to SFO

My questions:
1) for best fare we'll fly SFO to London (Heathrow); however flights from LHR to Dubrovnik are "steep", more economical options are out of Gatwick (EasyJet) or Stanstead (Ryan Air). Anyone have experience with those 2 budget carriers and suggestions on best way to get from LHR to either of those 2 airports?
2) Does the amount of times in each city I've allocated seem like too much/little based on highlighted sights below?

3) Car rental company suggestions & experiences? I assume there will be an additional charge to drop off the car in another city than Dubrovnik, but hopefully not as much as dropping off in another country.
4) Thoughts/suggestions on hiring a driver to take us to Montenegro?
5) Any experience with Flixbus? We'll likely use them to get from Istria to Trieste.
6) The Truffle Days Festival offers a train ride to go truffle hunting. Sounds like fun but may just a tourist thing. Anyone have any experience with this?

Highlighted sights:
- walled towns (Dubrovnik, Motovun)
- Krka & Plitvice Lakes National Park
- ancient ruins (Pula, Split [Diocletian Palace], Zadar, Lovrijenac Fortress)
- Julian Alps (Vrsic Pass), Lake Bohinj, Lake Bled, Vintgar Gorge)

Thanks in advance for your suggestions, they are definitely appreciated!

The only nonstop to Budapest is from Podgorica. For me, Montenegero is a lot more intriguing than Croatia. But if all you have for Budapest is two days, skip it and explore Montenegro. You will not regret it.

Ryan and Easy Jet are fine. Also look at flyig into Podgorica instead of Dubrovnik and see if that works for you. Then you can do your day trip in Montenegro on the way to Dubrovnik.

Based on your revised itinerary, you have 6 nights in Slovenia and then 2 days to get to munich or frankfurt to get home... I would suggest you add those train days to your main vacation and just fly from Zagreb or Ljubliana the night before to the hub you find your best flight home from... For example, this year we are taking the 6pm flight from Zagreb to Frankfurt, staying in the airport hotel (and having dinner at the Paulaner in the Squaire attached to the airport) and taking the morning non-stop home. With United to SFO, you have a variety of non-stop options. Getting the car in Dubrovnik, the drop off fee to Zagreb would be minimal and not much more for drop-off in Ljubljana. Look at Sixt rental cars. The whole drop the car, train to trieste/Lju seems unnecessarily complicated. As for Slovenia, the Soca Valley seems like a must-do and Bled is a stop-by. But travel tastes vary.

I would suggest staying in Split over Zadar. We’ve been to Split 3x and never tire of it. We’ve been to Zadar a few times more cause we have family there we visit. The last 2x we just stopped for a few hours to say hello and have lunch.
Not much to see in Zadar and the old town is very small. The sea organ everyone raves about is not visible, you can walk along the water front and hear the music. The Greeting to the Sun they also write about is just a solar panel on the walkway which lights up at night. The old cathedral is completely empty. Split is a better choice.

From looking at your revised itinerary I would suggest:
Ferry from Dubrovnik to Split (Croatia ferries.com)
Stay in Split. If going to Plitvice, you can skip Krka
Drive to Istria, stay in Rovinj, Poreč or Pula ( you can day trip to Motovun) these 3 cities are very close to each other.
Drive to Plitvice
Drive to Ljubljana, drive to Lake Bled for day trip, short, easy drive.
Drive to Zagreb, return car, fly home.