I've been reading lots of posts about most of the highlights in Croatia that most folks seem to have on their favorites lists. But a consistent theme seems to be how overcrowded/long lines there are in July and August. We are stuck going mid July to mid August (kids schedules) so can't really alter that. So my question is---are the crowds really so bad at most of the "highlight" locations, that it really is a drawback to go during that time of year (with other tourists and cruise ships)? If lines and crowds are that bad, I'm reconsidering whether we want to go there that time of year. We went to Scotland this past summer (just as a comparison) and other than the big cities (Glasgow and Edinburgh) really no crowds at all, and we were all over Scotland, and I have to say no crowds makes things hugely more enjoyable (driving around easier, no lines, no crowds at many sites, etc). My original plan was one month, from southern Croatia, up the coast, through Slovenia, then into northern Italy (Dolomites, Lake Como area, and Venice), but the issues about crowds this time of year has me thinking harder about this plan. I expected places like Venice/Lake Como to be crowded then, but if we are faced with crowds at most stops in Croatia, kind of cuts down on the fun. Thoughts?
Everyone hits the beaches during the summer so traffic will be snarled along the coast. I went to Croatia at end of August into early September and could probably not stand any more crowds than at that time, especially in Dubrovnik. It's also very humid there. I wouldn't go in the summer personally, but then again, I wouldn't go anywhere in Europe in the summer (except Scandinavia). Croatia probably didn't get too much tourism initially (once upon a time) but it's grown in popularity like gangbusters, so yes, to avoid crowds, don't go in the summer. On the other hand, you can certainly go off the beaten path and not visit the overtouristed spots and still have a great time...it's a beautiful place and I'm sure can be enjoyed without going to the top ten tourist sites.
If you have interest in the interior, it could be an amazing trip. Croatia has many national parks. But if you want to go to the coast then you will be sharing it with lots of others. I'd stick with your plan but concentrate on northern Italy, Slovenia, and northern Croatia.
The thing about Croatia is due to the shape of the country, a large amount of the attractions are along the coast and on islands. Those areas close down in cooler weather because there is not enough tourism to support them so they get crowded during the warmer months. So if you want to see those things that's when you need to go, crowds or no crowds. There are some areas such as Dubrovnik or Plitvice that you could visit offseason if those are your main interest. Over time Croatia is only going to get more popular and crowded. We went in September and the crowds were not too bad but it sounds like you are limited to when you can go. So GO and have fun!
I hate cold weather so end up doing a lot of traveling during the summer. It's a trade-off. What works for me is making a special effort to get to attractive places that are not so prominently mentioned in the tourist literature. That allows a respite from the hordes. A trip that covered just Venice, mid-day Plitvice, Split, Dubrovnik and one of the most-visited islands would be a bit much.
You can vastly improve your enjoyment of Plitvice by spending the night before your visit right near the park and getting a very early start the next day. It also might help just a bit to avoid the weekend. But starting early to get ahead of the day-trippers is critical.
The Istrian coastal towns of Rovinj and Porec were both mobbed when I was there in late August 2015. Still pretty, but they pushed my limits. The smaller coastal town of Vrsac was just fine (though small enough you probably would only need a couple of hours there), and the interior gets comparatively little traffic. I doubt that there's much to be done about Dubrovnik (which I haven't been to for decades), though I suppose hitting it on a day when there aren't a bunch of large ships in port would be somewhat helpful.
Croatia's such a lovely country, it would be a shame to miss it. Being in the high-volume towns very early and late (when the cruise passengers aren't there) would be helpful.
Nearby Montenegro is really great and not particularly overrun outside Kotor and Budva. Kotor was pretty quiet at the beginning of September on a day when there was no monster ship in port. The next day--Bam! I ran into comparatively few tourists in Ulcinj, Herceg-Novi and Cetinje. And with a car you could see the beautiful mountainous areas.
Be aware that Ljubljana is also very popular, and there will be many tourists in the old town. If you get beyond the core of the area right along the river, there will be far fewer visitors except up at the castle. It's probably a popular weekend destination, so perhaps scheduling the city for mid-week would be somewhat helpful.
The places I've visited in or near your proposed area where I didn't feel as if I was tripping over other tourists everywhere I went included Padova, Vicenza, Bologna and the Dolomites, though none of those is unvisited. There are other interesting cities in NE Italy, but I haven't seen them myself and can't comment on how dense they are with tourists in mid-summer.
The rule of thumb is to avoid summer weekends, be it for long distance or ferry travelling or for visiting the most crowd-prone highlights. Try to spend the weekend off the beaten path or use it for beach days, etc. Generally, though, while there will be people, the level of crowdedness in most places will be far from the swarms of people you may encounter in Venice, Pisa, etc.
Dubrovnik will be crowded at Venice levels, no getting around that, but it can be minimized by checking the cruise ship schedule and visiting on a low-activity day. If you want to avoid crowds and include an island in your itinerary, then by all means go anywhere except Hvar or Korcula. Most islands are full of people in the summer, but Hvar and Korcula especially so, as there are also large numbers of overseas tourists on top of the usual European holidaymakers. There are other options you can consider, such as Vis or Lastovo in the Split area (Southern Dalmatia), or better yet, go to an island further north. The Kornati NP is a lovely Robinson Crusoe type experience and most smaller islands in the Zadar area (Nothern Dalmatia) are calm. The larger islands in the Kvarner (northernmost island group) get crowded, too, but if you avoid Rab and Pag and stay somewhere on Cres, for example, you'll be able to have a nice time without too many people around.
In Slovenia, central Ljubljana will be full of tourists, but nowhere near the Venice or Dubrovnik levels of crowdedness, not at all, it's all still very relaxed - full disclosure: it's where I live. Lake Bled will be the one Slovenian destination that will be full of people, Piran on the coast will have quite a few, too. If you wish to see some of the caves, opt for the Skocjan Caves, which are wilder and less touristy than the Postojna Caves but of the same caliber in terms of quality. Nothing in Slovenia gets as crowded as Venice or most other Italian highlights, though, so I wouldn't really worry about this part of the itinerary.
The northeastern part of Italy (the part of Italy to the north and east of Verona-Padua-Venice) is an interesting region that doesn't get as many tourists as some other Italian regions, with the exception of Verona and Venice. I always find my visits to this part of Italy relaxing in the summer, unlike say Florence and Rome, which are absolutely mobbed at that time of year.
Edit: Corrected city in last paragraph.
Dejan makes a great point--if you just want scenic beach time without seeing the powerhouse places like Dubrovnik, Split, etc., then the northern islands are a perfect choice and fit with your itinerary of northern Italy and Slovenia. There may even be a ferry to Losinj from Venice at certain times of year. (Check Venezia Lines.) While there will be plenty of people on these coastlines, I think you can safely assume it will be in lesser amounts and consist of European families. Definitely something to consider. Easy for me to say after having already visited southern Dalmatia, but the lesser known islands are just as beautiful. The real appeal to me in Croatia is the natural beauty.
If you decide to go have a strategy in place to deal with crowds and heat.
Check cruise ship schedule for the relevant cities and dates.
Book centrally located hotels that are easy to get back to for a siesta.
Go early and late to avoid peak crowds and cruisers.
Grab lunch early to have a wider choice and get in before the restaurant fills.
If heat or the crowds get to be too much stop at a cafe. Have a gelato or ice coffee (coffee with a scoop of gelato in it). Relax and people watch.
Keep on traveling!
Sorry I couldn't resist.