Piran or Izola or Trieste ?

Piran because it is beautiful. Trieste is a big port city - meh. Izola is very small.

I loved Piran, but if you are already staying five nights in Rovinj, will Piran feel too similar? The towns are both in Istria (both sill bilingual, formerly part of Italy), just one is in Slovenia, one in Croatia.

Trieste is a big noisy Italian town. It has some charm, but nothing like the small-town charm you'll find in Piran and Rovinj.

Do you have to stay on the coast? What about another town inland? Or maybe a few more nights in Ljubljana? (How much time do you have there?) Although Ljubljana is small and doesn't take much time to see, it is lovely and vibrant and makes a great base to explore nearby towns.

My thought was the same as Andrew's. That would be an awful lot of small-coastal-town time for me. However, if you're traveling during the summer and looking for a lot of beach time, that would be different. Me, I would choose Zagreb in a heartbeat after five days in Rovinj (and I do really like Istria). I've never seen Izola. I think I'd enjoy Trieste, but I've only passed through there very briefly.

Our current schedule is:
Venezia - 3 days (ferry to)
Rovinj - 5 (bus to)
Piran, Izola, or Trieste - 3 (side trip to Lipica Stud Farm and Škocjan Caves) (train to)
Ljubljana - 6 (side trip to Bled) (plane to)
Wien - 5 (side trip to Bratislava)

I am open to any ideas for improvement.

I had ruled out Zagreb because I cannot see how to get from Rovinj using the train. We are only using the bus from Rovinj at present because we have to. My wife HATES buses.

I had ruled out Zagreb because I cannot see how to get from Rovinj using the train. We are only using the bus from Rovinj at present because we have to. My wife HATES buses.

Me to!!!

You could get a bus to Rijeka and train from there to Zagreb, but that would be a long day. (About five hours just from Rijeka to Zagreb.)

How about renting a car for a few days? Driving is easy in Croatia and cheap. Rent the car in Rovinj (you might even want it before you leave so you can explore some of the interior hill towns within Istria, drop it in Zagreb, train to Ljubljana. On the way, you could stop in the amazing Plitvice Lakes National Park, one of the highlights of Croatia.

We changed a lot.

Are now:
flying into Bologna, 4 nights
train to Venezia, 3 nights
train/shuttle to Piran, 5 nights
train to Ljubljana, 6 nights
train to Wien, 4 nights, fly home

I really appreciate all the advice as it helped solidify our travel plans. Our original goal was to spend a lot of time in Slovenija.

Bonus: Only one currency to deal with.

I like that trip. Really good variety.