Croatia/Slovenia/Venice

I love Venice but totally avoid it in the summer months. Could you possibly start there in May, then ferry to Croatia, and end in Slovenia? If you with to visit the mountain regions of Slovenia it would be best to leave it til last anyway.

Have you looked at the RS Best of the Adriatic tour on this site? That is a good option, but, if not, you will have a template for planning your trip to Slovenia and Croatia. The tour ends in Dubrovnik, and you can make your way to Venice. Another option would be Venice first, fly to Lubjliana. Or, save Venice for another trip and drive from Dubrovnik to the Bay of Kotor in Montenegro. We had a VRBO there and really enjoyed it.

Here's one website with information on ships in port: Cruise TT. There are others (just Google "cruise ships in port"), and it would probably be a good idea to check multiple sources, since this factor can seriously affect your enjoyment of some coastal cities.

You need to get a guidebook that covers Slovenia and especially Croatia, to help you winnow the list of possible destinations down to something more manageable. A good guidebook targeting budget or mid-market travelers (i.e., those who may not necessarily use rental cars and taxis all the time) will provide useful information on transportation links that will help you determine what is likely to be feasible in a given amount of time. Then you can look further for more details and more current information.

I don't have recent experience visiting any Croatian islands, but here are two online resources that can help with logistics:

Rome2Rio.com is a good first step toward determining whether there is bus service and/or ferry service and very roughly how long it might take to move from Point A to Point B. The information is not terribly precise (don't trust the prices!), but if you keep drilling down, you will usually eventually arrive at a link to the website of the appropriate bus or ferry company, where you may find schedule information. If not, Google "ferry Split to Hvar", etc.

BusCroatia.com is a rather comprehensive source for information on inter-city buses, including some that connect to Slovenia (of which there are not many). However, it is essential to verify the information once you reach Croatia. I always tried to stop by the local bus station a day or two ahead of time to look at the posted schedules, though I found the local tourist offices quite helpful if they were terribly busy at the time (could be an issue in Dubrovnik). This is especially important for buses that cross international borders, which you should book ahead of time since they sometimes sell out. The young people in both Croatia and Slovenia tend to speak English quite well, so it probably won't be as difficult to confirm schedule details as you might imagine. Do not, however, expect to get detailed information on Croatian buses while you are in Slovenia, or vice versa.

Bus schedules often vary more by day of week than from month to month, so if you find limited information available at this point for next summer, check the schedule for the day of the week you expect to be traveling, but use a date in December. Sunday schedules--which often apply to holidays as well--can be quite limited, so that's a good day to plan to explore the city you're currently staying in. And of course do investigate the holiday calendar for both countries.

It has been mentioned on this forum that lodgings on the Croatian islands often require at least a three-night stay, at least during the busy season. I don't know whether you'll run into that during your anticipated travel period. You could do some preliminary checking (trying 1-night bookings vs. 3-night bookings) on booking.com, to see whether many more options show up for the longer stays.

I'll close with my usual warning about Plitvice Lakes National Park: This is one of Europe's top sights, but everyone knows it, so it is slammed with thousands of tourists every peak-season day in the same way Dubrovnik is. And the walkways in the park are a heckuva lot narrower than the streets in Dubrovnik. To have a really enjoyable time at Plitvice, you must spend the night before your visit very near the park and get an early start the next morning. Others have found that it can be nice to visit the park very late in the afternoon as well, after all the day-trippers have left. Do not try to do this as a day-trip from Split or Zagreb! One night at the park should be sufficient.

You have some excellent advice.

Thoughts:
We spent 2 weeks last May driving through Slovenia, Croatia, Bosnia, and Montenegro. We rented through Sixt and paid 260 USD for the 2 weeks. We determined that was the easiest way to connect all of the points we wanted to go to without figuring out the bus system.

We took the car ferry - there are 3 per day - from Split to Hvar and then drove the car from the ferry to Drvenik and down into Dubrovnik, where we dropped the car upon arrival into Dubrovnik - you don't need the car in the city. Some posters drop their car in Split, ferry to the islands, and ferry down into Dubrovnik.

I would have loved to ferry to other islands, mainly Korcula, because I love beaches, but we had decided on 2 nights on Hvar - so we had 3 days on Hvar. We loved the time there, had a delightful apartment balcony overlooking the marina, and we thought the island was quiet.

If you go to Plitvice, drive there and spend the night. You can go check out the park and take sunset photos the evening before. Then, arrive EARLY the next morning. We walked every walkway, stopping for numerous photos, and still left the park by 2 and drove into Split for that evening.