First, you are going to love your trip. We did a very similar route back in 2007 and it is still one of our favorite trips (if not, the favorite).

With regard to Split and Dubrovnik, we much prefered Split to Dubrovnik. For us, there was more action, shopping and other urban-type things going on in Split than Dubrovnik - and you still had the harbor/beach (plus the Split boardwalk area). Dubrovnik is very picturesque and quaint, so as for splitting your time up, it will depend on what you prefer. Also, I'm not sure if there is a reason you prefer flying or taking a bus to Split/Dubrovnik, but if your rental agency has an office in Dubrovnik, you should be able to drop the car there and not have to pay a drop fee since you would still be dropping in Croatia. You'd have to confirm with your rental agency, but it usually has to do with dropping in a different country, not city.

As for your specific questions:

1. We rented through National and had no problem taking the car into Slovenia. We also took it into Montenegro and Bosnia without a fee or issue. I am not sure if the low-cost agencies are the same, but, National, at least, would fit what you're looking for in terms of driving into Slovenia.
2. We have not made it to Istria on either Croatia trip, so I cannot comment.
3. We thought Plitvice was simply amazing. My husband is from out west, so we have been to the Grand Canyon, Yellowstone, Yosemite, the Badlands, etc. - and Plitvice was still breathtaking. We stayed at a B&B just outside the park and proved wonderful. It was called Guesthouse Knezevic.
4. Depending on which island you are looking for, there are ferries that leave from both Split and Dubrovnik to some of the islands.
5. Other than what I mentioned above, here a few additional comments:
a. If you are already in Plitvice (and are not forced to return your car to Zagreb) you are not terribly far from Split, so you could continue on with your car from there, rather than circling back.
b. Although you didn't mention it, if you want to visit NW Slovenia (Kobarid area), there is a great "car train" that will take you from Bohinj to Most na Soci. It is what it sounds like - a train that pulls basically flat-bed railcars and you drive your car onto the flat-bed. There are low rails that pop up and the train takes you through the mountains, cutting your driving time in half, at least (the train took an hour). We thought it was a cool way to get around - especially my husband, since he was doing all of the driving. Just throwing it out there . . .
c. In Dubrovnik, there is a cool bar just outside the old town, on the cliffs overlooking old town and the city walls. A very cool view, but no railings. The bar is called Buza 2. Basic directions - Follow the city walls around at the very south of the old town on Od Margarite. Look out for a doorway in the wall, with a wooden sign saying “Cold Drinks.” Enter there.
d. You mentioned not getting to see Roman ruins that often - Split has Diocletian's Palace, so you can see the digs of an old Roman Emperor. Also, Solin (aka, Salona) is close by and has ruins from a decent sized Roman settlement. We stayed actually stayed a little outside Split - in Trogir - at an apartment called the Yellow Apartment. Since we had a car, it was a nice way to save a bit of money, while still staying at a nice B&B. It isn't right in Trogir, but for us, it worked - especially since we used the accomodation savings here to upgrade our normal accomodations in Zagreb.
e. We found driving times in Northern Croatia and Slovenia to be much less than we had allotted, but driving times were longer in Dalmatia.
f. There is a neat little town between Zagreb and Ljubljana called Samobor, if you are looking for something outside Zagreb. A quiant town square and about a 10-15 minute hike to the remains of the old Samobor castle.
g. A little further afield is Varazdin - about 50 miles or so north of Zagreb. A cool city center and castle complex.

I hope this helps.

Sounds like a nice trip!

If you are interested in visiting one or more of the south Dalmatian islands, you might want to shift a day or two from the north to the south. Also, while in Istria you might want to consider a brief stop in Porec. The Roman ruins there are minimal, but the Euphrasian Basilica is amazing.

Akrobat's suggestion that you drive from Plitvice to Split and drop the car off there is worth considering.

I thought Plitvice was astoundingly beautiful - and I've seen many beautiful waterfalls and lakes.

Another vote for the Buza Bar in Dubrovnik. Search this site for more raves about it!

Enjoy!

I would choose Rovinj over Piran - the city is larger and there is more to offer.
Don't miss Plitvice - it is simply beautiful, but make sure you spend time early in the morning or late in the day when there are fewer people in the park.

Hi there,to answer a couple of your questions -
1. We hired a car with Hertz in Trieste, Italy and drove it into Slovenia, then Croatia and back again. There were no problems with doing this but we did have to tell them upfront as we needed certain paperwork for the other countries. You also need a vignette to drive on the highways in Slovenia. It's like a sticker you buy then put on the windscreen. And you probably know this already but if you pick up and drop off the car in the same country it's much cheaper.

2. We stayed in both Piran and Rovinj. Both are lovely but I would vote for Rovinj, it's bigger with more to see and is very picturesque. Piran is pretty but very small. Visit both if you have time.

4. It may not be what you are after but we did a half day trip to Lokrum, an island about 30 minutes by ferry from Dubrovnik. There isn't much on the island, a cafe and toilets, but you can walk, explore and swim. Getting there was cheap and easy.

I believe staying in a Sobe (house with rooms for rent) would be about the cheapest way of getting accommodation in Croatia. If you walk around the streets you'll notice the blue Sobe signs or ask at a travel agent or bar.

Have a great trip, we've been twice to Croatia and loved it. Saw a tiny bit of Slovenia and that was good too.

Kay

I rented a car from Uni-rent at the Zagreb airport. They allow border crossings and they were very reasonable.
I really liked Rovinj but did not see Piran so I cannot compare. But parking in Rovinj can be tricky.
I really was underwhelmed by Plitvice. Just one opinion.

We parked in a big open air carpark near the water, about 5-10 mins walk from the centre of Rovinj. It was a bit pricey though so if your accommodation can give you free parking I'd take that.

Kay

Can't help you with the north, but the Split-Dubrovnik part we did last summer and loved it. We did four days in Dubrovnik but took day trips on two days (Mostar and Montenegro), two nights on the island of Korcula on the way to Split where we had two more nights. We were also there in July and Dubrovnik was crowded - among the most crowded places I've been. But it was still "worth" it and everywhere else was not bad at all. We stayed in sobes and they were great and very economical.

I did a trip report - there are links to my photos and to the websites of the sobes we stayed in. The trip was part of a three week trip to Italy also so you need to scroll down about half way in the report to get to the Croatia part.

Just wanted to thank everyone for the useful info, especially the comments on Plitvice & suggestions re car rentals.
I had just about despaired of this trip, as air tickets had continued high, but just today I checked & found tickets into Zagreb & out of Dubrovnik for slightly under $1400 Cdn.
Happy dance! Song of joy! We're actually booked and now we get to start organizing itinerary & hotels & a car.
Can you tell I'm excited??

bookmarking

Second on the advice of using Sobe. Hotels are much more expensive. Our Plitvice side trip was enhanced by our sobe adventure. "Mama" and "Popee" rent out their bedroom during the tourist season and stay in a modest little guest room. Mama doted on my daughter as she tried to catch a chicken, while Papee sat my husband & me down with some homemade hootch and a Croatian/English dictionary. Oh, and Plitvice was beautiful too. What I wasn't prepared for were all the abandoned Serbian houses dotted with bullet holes. My husband said it was like seeing the stitches of war rather than the scars. It was powerful scenery all the way around.

Most of our other sobe rentals were like apartments. Mama and Popee were special.

We weren't that crazy about Split. Loved Korcula and extended our stay there (wonderful sobe there. very popular. Villa DePolo). You wouldn't need a car.

That's great, nfldbeothuk!

Sounds a great trip.

I've been to the Dalmatian coast several times now, and also into Bosnia and Herzegovina twice. I always rent my car through www.economycarrentals.com for the best rates. They are agents, so you get put with other rental companies, but the prices include full CDW and I've always found them most helpful. If you order the cross-border card when you make your online booking often you are not charged any extra. I would rent with them again without any hesitation.

Good luck, and please post a trip report as I love this part of the world and heading further Croatia is now high on my to-go list! I want to read about your Istria and Slovenia experiences!

We live near lakes and mountains with plenty of waterfalls but still found Plitvice very beautiful; there's a reason why it's a World Heritage Sight. The limestone in the soil turns the water a special pale aqua blue. Our sobe host in Dubrovnik, however, said he preferred Krka.

And we too loved Korcula. There's something magical about that small and perfect place. It supposedly was Marco Polo's birthplace.

I actually think, given your interests, you could omit Slovenia (though I kick myself for not having time to see it). Istria has a lot of Venetian/Italian ambiance. And there's an almost complete Roman amphitheater in Pula, also a few columns etc., in what is otherwise an unmemorable town.

As for parking in Rovinj, we parked for free on back streets -- but this was later September.

Make that World Heritage Site/Sight. Aargh!

If you drive up to Bled in Slovenia, you can then drive to Lake Bohinj and take the car-train to Most Na Soci. From there you can go to Tolmin (visit Tolmin Gorge)and drive south to the coast and Piran (go to the Skocjan Caves on the way).
However, if the weather is nice you may prefer to go north from Tolmin into the Triglav National Park (Bovec-Trenta-Vrsic). This is the Julian Alps area and the scenery is spectacular. There are a lot of nice hikes and outdoor activities such as rafting.
All of the towns on this route have very good tourist info offices and we found excellent and reasonable B&Bs (Sobe) by inquiring at these offices. They also provide excellent maps, brochures and advice about the attractions in each area.

My husband and I have been thinking of a fall trip to Slovenia and Istria. Yesterday when I checked different airfare options I discovered we could fly from Boston to Ljubljana in late September for $864 which seems very reasonable. We were thinking of seeing Bled and Piran in Slovenia plus some of Istria in about 10 days but as usual there are so many tempting places to visit it is hard to choose.

We may try to combine car and train travel as my husband is a big train buff.

Bookmark-we are planning a trip to Croatia & possible Slovenia so this is very helpful. Thank you everyone!