Venice --> Croatia --> Slovenia --> Venice

Rental car--- Input your ( potential) desired pick up point(s) and Venice as the drop off and see if you get a 'solution' It will show the cost, if it is even possible.

With a noonish flight from Venice you will need to be at the airport no later than 10 AM, probably earlier if the flight is to the USA, so suggest you make Venice your last stop. Otherwise traveling there from afar is a high risk proposition.

Googlemaps shows the drive time at 2 hrs 10 minutes, which is likely optimistic. Adding time to refuel the car before turn, doing the turn in etc., and you are looking at a 6AM departure( wheels rolling) from Soca Valley. You may be OK with a 4AM wake up, but it would be my last choice option.

Good luck!

I went to Slovenia and Croatia in May 2015 for three weeks. First off, you are packing a lot into two weeks, and IMO missing some of the best of Croatia (Split, Dubrovnik, Hvar). We flew into Dubrovnik and out of Zagreb. Rented a car the whole time (driving is very easy there). From what I have read here (travel forum), getting to Plitvice is really hard via public transit (and you don't want to skip Plitvice as it is awesome). Also, keep in mind that your travel times may often take longer by bus/train than by car (due to need to get there early to not miss the bus/train, etc...), thereby making your already jam-packed schedule more crowded. (Clearly, I have a bias about going where I want to go, when I want to get there...haha). I dk if you already have your plane tickets (if so, not much you can do). You may want to look at some of the cheap inter-Europe carriers (RyanAir, German Wings, etc) as they may have really quick, relatively cheap flights from Italy to Zagreb or Ljubljana, saving you time on the ferry. I think there was also a car service that would drive you (don't remember what it was called).

They are all beautiful places and you will have a great time no matter what. I would recommend posting this to the Croatia and the Slovenia forums to get more specifics. There was lots about the ferries and public transport there.

PS. I did not find Pula or Zadar worth more than a day trip. Check Rovinj and Motovun and (as I said before) Split.

Renting a car in Venice would certainly be the most practical way to do this if you need to fly out of Venice. What about say flying out of Zagreb - fly an open jaw into Venice out of Zagreb? (Or even Ljubljana, but it tends to have poorer connections in my experience, but that depends on your city of origin.)

If you need to fly into/out of Venice and don't want a car the entire time, you could take a shuttle to Ljubljana from Venice, spend a night or two in Ljubljana, then rent a car as you leave, drive to Bled, Bohinj, then to Zagreb, Plitvice, islands, Istria, etc....and drop the car off near Koper or Piran, Slovenia, and spend the last night there (Piran is a lovely seaside town). Then take an early morning transfer with a company like GoOpti on to the Venice airport. (Between Ljubljana and Venice and its airports there's also a company called DRD with scheduled daily service.)

But all of that extra hassle merely saves you having to drive (jetlagged?) when you arrive in Venice - unless you spend those first few nights in Venice anyway.

GoOpti (a Slovenian company) will also transfer you between Venice and Istria - check their website for quotes (I have never used them but hear really good things about them). The ferry from Venice is in the evening I think - may not be the ideal time, not sure. If you start after Venice in Croatian Istria, you could rent a car there and return it say in Zagreb if you can fly out of there.

Venice is a lovely city, but if you really aren't interested in spending time there, it might make sense to fly into Zagreb or Ljubljana to start. Sometimes you can fly to Amsterdam or Paris or something and book your own flight on even the same day (if there's enough time in between - and depending on your tolerance of risk).

As for public transportation: Croatia and Slovenia have limited train service but decent bus service. Try the website Rome2Rio to find the way between places (not always complete info there but it's a good start). Try the website GetByBus.com for Croatian bus connections. But yes, there are places where having your own wheels will make more sense.

I suggest putting down a proposed itinerary in list form (where and how many nights?) and then look for the most practical way to connect them. I have a few of my Croatia and Slovenia trip reports online - send me a PM if want a link to read them.

Open jaw is always a good idea. But, be aware that renting a car in one country and returning it in another will either have a high surcharge or not be allowed.
Driving to another country (or multiple countries) then returning the car in the same country you picked it up in, even in another location in the same country, is usually no problem.

I traveled to many of your Croatian destinations, Ljubljana and Lakes Bled/Bohinj in 2015, all by public transportation. Yes, things will be more efficient with a car, and I think your Slovenian targets are not doable in the time you have without a car (if even then), though Ljubljana-Bled-Bohinj is OK by bus. Zagreb to Plitvice is also fine by bus, in my experience, though it does somewhat constrain your choice of lodging in the Plitvice area. You can also go on by bus to Split and I would guess also to Zadar, though I'm not 100% sure of that. Just be sure you do spend a night near the park so you get to see it before the day-trippers arrive. It's a much less enjoyable experience if you arrive later in the morning, believe me.

There's bus service in Istria connecting Porec-Rovinj-Pula frequently enough that I visited Porec one day from Rovinj with a stopover in the cute village of Vrsar. I didn't go to Pula (which I gather is physically less attractive than the others but has some Roman stuff). Basically, Rovinj looks medieval, with lots of stone and windy streets. Porec has colorfully painted buildings and is flatter, with a street plan that seemed a bit more grid-like. Try to see both since they are so different.

You can also get to Rijeka by bus from the Istrian coastal towns, though I don't know how frequently those buses run.

The main issue in doing Istria without a car is that it's difficult (probably impossible) to see the two most-mentioned interior towns of Motovun and Groznjan on the same day by bus. I avoided that problem by pounding the pavement in Rovinj until I found a travel agency selling a one-day bus tour through the interior. In 2015 it ran once a week on Wednesdays. Next year, who knows? It was called "Inland Istria" and was worth the money to me (K445 / US $71 per person in 2015).

I took a bus from Ljubljana to Rovinj, but I think it ran only once a day. Certainly getting to Slovenia from Zagreb is much simpler. In both cities the bus and train stations are nearby, but the Zagreb bus station is definitely a longer walk from the old town. I like Zagreb a lot for its historic district and variety of museums; naïve art and broken relationships are both great, plus there are more traditional options. Zadar is good for a one-night stopover. Very pretty historic district and a couple of contemporary art monuments along the waterfront, one of which you need to see after sunset.

I think you need to spend your last night in Venice, so I'd try very, very hard to put the Venice part of the trip at the end rather than the beginning. Anything else is going to be stressful, I think. At least, I would be stressed.

I haven't tried to check the schedule for the Venice-Pula ferry. Folks on this forum have been happy with GoOpti shuttle service between Trieste and Slovenia, but arranging that on your arrival day would be risky. People say Trieste is interesting. I'd consider spending my first night in Trieste so I could arrange onward transportation on Day 2, without worrying about a delayed flight. Although I do like Zadar, if you had to give it up for a night in Trieste, I don't think it would be awful.