Hi!
I was thrilled to inadvertently come across Rick Steve's website.
I've never been to Europe and may never go again so want to fit as much in as I can in 3 weeks and at a stretch 4 weeks.
Is it feasible to visit Dubrovnik, Split, Hvar, Plitvice Lakes, Zagreb and then to Venice, Florence and Rome?
I find it more challenging to plan a trip linking two countries, but it can be done. I always start by writing down each day, something like this:
Arrive Dubrovnik
2-3 nights
catamaran to Hvar
1-2 nights
catamaran to Split
1-2 nights
bus to Plitvice
1 night
bus to Zagreb
1 night
Then, fly to whichever Italian city has direct fights, travel between Italian cities by train. You would want no fewer than three nights per place in Italy.
This is a fast paced itinerary, and I would add more time to every place that you can--these are just minimums.
Use Google to search flights and transport between places.
Obviously, you can do different orders and part of that will depend on what time of year--play around with all of the combinations to see what works for you. Good luck!
It is. In my experience it is extremely easy to fill time in Europe, where it's rare to travel 10 miles without passing a place of interest. Therefore, I'd shoot for as long a trip as you can manage. As you dig into guidebooks, you will surely find more and more places you'd like to go. The shorter your trip, the more painful the omissions will be.
The Italian cities--especially Rome--will be more time-consuming individually because of the sheer number of sights. They are among the most popular destinations in Europe for a reason. Transportation from place to place will be somewhat slower in Croatia because you'll be on buses and ferries vs. the express trains in Italy. The link between Zagreb and Venice is not fast; by train it will be 9 to 10 hours. Buses are scheduled to be about 3 hours faster, but I don't know how reliable they are. They will be nice, modern buses.
I'd recommend getting some guidebooks (likely available from your library). Rick's books are very good on how-to-travel topics as well as providing in-depth information on what he considers to be the top sights. There is some summary sightseeing information on this very website. Go here, click on a city, then on At a Glance.
https://www.ricksteves.com/europe/italy -- for all three Italian cities
https://www.ricksteves.com/europe/croatia -- detailed info available only for Dubrovnik
You can watch Rick's TV shows here: https://www.ricksteves.com/watch-read-listen/video/tv-show. The Croatia show is listed under "Central and Eastern Europe". Many of the videos are quite old, so details may have changed.