Trieste

I don't know about Trieste but I would stop in Lipica at the Lippizaner horse farm!

You have to pass by it anyhow. So you might as well pay a visit.
http://www.turismofvg.it/Locality/Trieste

Why not just take the ferry?

Trieste is a beauty and certainly worth a stop. The 19th-century city centre is almost unchanged since the days it was the sea port of the Austrain-Hungarian empire. Have a coffee or a lunch along the Canal Grande or on the Piazza Unita d'Italia.

I was in Trieste for an overnight, planning for a few days there in 2018. Very nice city, not heavily touristed.

Also really liked Trieste. It is a flat city with lots of pedestrian only streets, which was great for my limited mobility. There are great apertivo and gelato places near the main square. Do you have your own car? We took a bus from Rovinj to Trieste and it was easy, cheap and very scenic.

I spent a couple days there last year, and I agree with others that it is an attractive city. I have no personal experience with this, since I traveled there by train, but I've read that the traffic can be quite bad. Just something to look into further before you make your decision.