Slovenia/Croatia - sooo much to do with so little time!

Pula has some interesting Roman ruins, but it is among my least favorite stops in Slovenia and Croatia. A couple of hours there was plenty for me - and I wouldn't have minded skipping it entirely. By contrast, I loved little Rovinj where I spent a few nights nearby. And I enjoyed a few hill towns. But I had seen better Roman ruins in Italy.

But personally, I would skip Croatia entirely. It's just not worth the extra driving time with such limited time to explore. Stick to Slovenia and you can slow down a little (but not much!). Looks like you'd have five nights. I might do them this way:

Ljubljana (2 nights)
Rent a car, drive to Bled
Bled (2 nights) (base to explore Bled, Triglav, Lake Bohinj)
Drive the Vrsic pass "Julian Alps" loop, down into the Soca Valley, and drive all the way to Piran.
Drop the car near Piran (Portoroz or Koper) and spend the night.
Take a bus or shuttle service (e.g. GoOpti) from Piran to Trieste, and take a train from there to Florence.

Two nights in Ljubljana isn't really much if you arrive mid-day the first day and leave early the third day - basically 1.5 days. It's not a big city and doesn't require tons of time, but it is lovely and charming and not nearly as touristy as other places on your agenda.

Piran is a lovely little town with a beach, kind of (I went swimming there in the ocean but it was late September - water may be a little chilly still in May). You could extend your last day there a little if you want to dwell and get mid-afternoon shuttle to Trieste and a later train into Florence, if that would work.

If you want to skip the flight from Vienna to Ljubljana, you could get a direct train (about six hours) from Vienna to Ljubljana that arrives just a tad later - at 14:06 but in the city center. It's certainly a lot less hassle than getting from Vienna to to one airport, go through security, fly, then get back to the city center of Ljubljana. Alternately, you might consider a night train (not something I have ever done, but some people swear by them) that would get you into Ljubljana really early, by about 6am.

I don't always agree with Andrew H's advice on Slovenia and Croatia, but I think he's hit the nail on the head here.

I believe there might be a bus from Vienna to Ljubljana that takes under 5 hours, quicker than the train Andrew suggests. But either way, train or bus might be more convenient and not much longer than flying when you consider the time it takes to get from city center to airport and then from airport to city center.

Most rental cars are not allowed on ferries.

I vote with the others. I love Istria, but seeing one town well is better than running through three. And Ljubljana is a really, really nice place simple to be. I understand the draw of Roman ruins in Pula (a town I have not been to, despite 3 trips to Istria), but I think it's significant that no one on this forum has said anything really positive about Pula. Everyone who has been there is sort of "meh" about it, so I don't think it's a place you should stretch hard to see when it means cutting other very nice destinations painfully short.

This reads as a very aggressive trip, but you're clearly decades younger than I am.

One key point: You list 3.5 days in Prague, but you have nothing like that:

May 22: Arrive in Prague after a probably-sleepless overnight flight. Drop off luggage at what time? Not much true sightseeing that day if you're like me and zombified.
May 23: Prague
May 24: Prague
May 25: Leave for Vienna

I count that as two days plus some wandering-around/trying-to-stay-awake time on May 22.

Similarly, you don't have much more than four days for Slovenia/Croatia--but that is already being addressed.

Wanted to thank everyone for their insight. I have adjusted the itinerary accordingly and am excited to get on the road!
Many thanks!!