Is my itinerary too robust? Suggestions?

Definitely an aggressive pace. Maybe a little too aggressive (this from someone who usually moves very fast).

Some questions/observations...

• I suggest you write-off (don't count) your arrival day. Almost certainly, you will have a redeye overnight from NYC to somewhere in Europe (probably Frankfurt) then connect to Ljubljana. Although it's an easy, relaxed placed to arrive, unless you are very good at sleeping on flights, you're going to be wiped out that arrival day. I'd suggest you plan just a few hours walking around.
• You didn't specify how you're planning to get around. Renting a car? You will almost certainly get dinged with a major fee for picking up in Slovenia and dropping off in Croatia. You need to figure this out before you do any more planning, because it could have a major impact on either your budget or your itinerary (when I did a similar trip a few years ago, I rented one car for Slovenia, and a second in Croatia to beat this fee, but it was complicated and ate up some time - which you don't have in excess).
• First part of your trip seems aggressive but do-able assuming you have your own car, but...
• Check drive time from Ljubljana to Plitvice. I went a different way. Otherwise your Plitvice plan seems reasonable (similar to how I did it).
• Check drive time Plitvice to Trogir (I went a different way).
• I assume your "Trojir" is "Trogir". We stayed in Trogir and really liked it, but I'm not sure that most of a day there is needed. It's a cute old town and has its charms, but pretty small. We spent 1 night there and a few hours wandering seemed to be enough. Food alert: we had the best two meals of our trip at one restaurant in Trogir: "Pizzeria Kristian" (which is saying a lot - the food in Croatia was incredibly good). We had dinner there our first night, it was so good we asked if they opened for breakfast. They didn't serve breakfast but we were waiting there when they opened for lunch and ate there again. I'd go back to Croatia just to eat there. Not expensive, great service. Get the home-made pasta. I'm hungry now... Anyway, I'd give Trogir half a day (although you might want to time it so you can eat lunch and dinner there...). Plenty of other places/things to see around there so if you can shave half a day off Trogir, you can spend it elsewhere.
• We didn't care much for Split. I know, everyone raves over Split. It was completely jammed with crowds off cruise ships, the crowds really turned us off (and we were there in May, before the peak season...June would probably be worse).
• We also didn't fall in love with Hvar. We had a few hours there (between ferries) and I didn't like the vibe - it's obviously very popular and considered the place for the cool kids. I was ready to move on after just a few hours.
• We moved on to Korcula (the next island), which we absolutely loved. While Hvar seemed crass, tragically hip, and oh-so-full-of-itself, Korcula seemed even more pretty, but authentic, unspoiled, and charming. Tastes vary though, I know plenty of people think Hvar is great.
• Don't know anything about Pakleni Islands.
• Re another day in Dubrovnik versus a day trip to Montenegro: we spent 3 nights and one full day in Dubrovnik and that was enough. Be aware that the city's character (and your experience of it) will depend to a very large degree on how many cruise ships are in that day - and the same will be true of Kotor, over in Montenegro. We got lucky (or smart) and managed to spend our day in Dubrovnik on a day when there were either few or zero cruise ships. It was still crowded, but not horribly so all day everywhere (on the previous/next days, it was noticeably more crowded and the place really lost a lot of its charm). Kotor was wonderful - on a day with no cruise ships. An easy day trip (we rented a car for that from our pension's owner). We loved Kotor. Climb the fortifications behind town!

Hope this helps. Have fun!

Very helpful thank you. I'm aware of the car rental issues Slovenia/Croatia. As of now the plan is to hire a driver in Slovenia for the Slovenia travel portion, hire a driver to get to Plitvice and Trogir/Split; catamaran from Hvar to Dubrovnik and rent a car ourself for Dubrovnik to Montenegro. I'm beginning to think it's too robust too, and flying to Slovenia is more expensive than Croatia. Thinking maybe just do one or the other this trip. So maybe Slovenia & Plitvice and that's it and do Trojir/Split/Hvar (or Korcula)/Dubrovnik/Montenegro another time, or vice versa. Between those two options which would you do first, considering my primary interest is nature/beauty vs history. PS -- that pizza place in Trogir sounds very appealing!

Tough call. Nature...well, Slovenia has plenty (the mountains, Lake Bled, the caves) - but Croatia has Plitvice, and there's no denying the islands and coastal Croatia are utterly spectacular. There's more/deeper/visually more obvious history for the casual tourist in Croatia...

One potential consideration: it's my sense that Croatia has been very, very "discovered" in recent years (thank Oprah and Game of Thrones). It's become very popular, and in places it's just crowded. One might want to prioritize Croatia first, since the crowds are probably not going to thin out with the passage of time (think Cinque Terre). My sense was that Slovenia was not nearly as well-known as a major tourist destination and we never experienced serious crowds there...no cruise ships, no large organized tours all following the guide with the flag...so maybe Slovenia will keep its charms for a while.

FWIW we went in mid-to-late May. Technically, "high season" (which all the locals seemed intently focused on) was considered from June 1 to end of August. So we were there before "high season" and it showed - we saw lots of places that were clearly "gearing up" for what felt like a major onslaught that was coming soon. My sense was that a week or two (late May to early June) would make a big difference. And in some places, even in late May, it was already crowded enough that it turned me off (Split would probably have been great if all the damn people weren't in the way). Granted, I have a bad attitude about crowds, though, so if you're not as much of a misanthrope as me, it might not bother you...

FWIW, I'll weigh in as someone who hasn't been but am planning a 2-week trip there in September. I had originally planned on splitting my time between Slovenia and Croatia, but after doing my research (I, too, am more in to nature, and avoiding, when possible, crazy crowds), it now looks like I'll spend only 1 day/2nights in Plitvice for my Croatia time. Rest of time will all be in Slovenia (Ljubljana, Bohinj with day trip to Bled, drive Vrsic Pass, Logar Valley, and a spa town in southeast). I plan to rent a car for most of my time.

Shelly, sounds like a good plan. I'm still torn which way to go.

You are trying to cram in more than I would, given your timescale of 13 days, particularly the first portion.

Assume that day 1 will be a write off due to jet lag. Your first Monday is very hectic. It is best to visit Plitvice early in the morning before the bus trips arrive.

Croatian schoools start their long summer holidays from the middle of June, so places will be busier. Plitvice should be slightly less busy during the week rather than the weekend, but being June, it won't be quiet.

There is far more to the island of Hvar than Hvar town - personally, not being under 25 (by quite some margin!) I wouldn't contemplate staying in Hvar town this time of year as I value my sleep. I would consider Stari Grad or Jelsa instead or head to the next island, Korcula. Check online how busy Dubrovnik will be with cruise ships the day you are planning to spend there - you may want to swap your itinerary around and have your last day in the city, rather than travelling miles.

For only 11 days there, I would just focus on Croatia as there is plenty to see.

Yes, I'm starting to agree. Wasn't planning on staying in Hvar town but others have recommended Korcula as well so I'll look at that. I could go the first week in June instead - might that help all around? Someone else recommended staying outside of Dubrovnik to avoid the cruise crowds, say in Cavtat so that's a possibility too. Thanks for your input.

Someone else recommended staying outside of Dubrovnik to avoid the cruise crowds, say in Cavtat

I hear that a lot here, but I'm not convinced. First, at least for me, staying in the center of an old city is one of the main reasons I go to places like Dubrovnik. I'm sure Cavtat is pleasant (and less expensive), but being in the old center is a highlight for me. Second, if you essentially "day trip" into Dubrovnik (or other too-popular place), you are actually making things worse for yourself: the day-trip crowds are actually what ruins these places (for me), so why on earth would you want to go there when the mobs are worst, and not be there in the morning and evening when the day-trip crowds are not there (or at least not as bad), and the place is actually enjoyable?

IMHO, the way you "beat" mass tourism's over-crowding at many places in Europe is to sleep in the old center, get up early and enjoy the place for a few hours while you have it (relatively) to yourself, flee to somewhere else during the worst of the mid-day, day-tripper, cruise-ship-dump crowds, then come back in the afternoon after all the day-trippers have gone away, so you can enjoy the place for a few hours into the evening. The downsides of this approach are 1) you pay more to sleep in the old center (true...but you came here for this place, not one of its suburbs, so suck it up), and 2) during the worst of the mid-day crush, you are better off going someplace else (that's when it's nice to flee to a less popular place - maybe even a suburb - and there are usually places worth going/things worth doing outside the magnetic centers).

Again, depends on how much you want to avoid the crowds. For me, that's pretty important, and I plan my trips around that. Others have different priorities. Vive la difference.

All good points David.

To add some data to the crowds debate: Tourist numbers in Slovenia are growing at over 15% p.a. Numbers in what used to be off-the-beaten-path places are growing even faster. Overnight stays in the Soca Valley have jumped from 450,000 to 600,000 (+30%) in a single year. So if crowds were a consideration I would probably prioritize Slovenia as it's probably just a matter of years before it becomes crowded too, unfortunately.

Regarding the itinerary, I agree that it's aggressive. It would makes sense to recover from jet lag in Ljubljana in the beginning and only then head out towards the Alps, but I don't think it will make a large difference either way. And if you were to narrow the itinerary down to a smaller region, I wouldn't necessarily frame it as Croatia or Slovenia but rather as Southern Croatia (Dalmatia) or Slovenia and Northern Croatia (Istria & Plitvice).

Yes agree to all...am already cutting it back to one region. Thanks all.