I'm on a scouting trip quest where I try to jam in at least 2 countries on the same trip. I do some quick sight seeing in the old towns of cities ofc but I do it quickly. The main aim is mixing with locals and feeling what living there would be like and in my experience taking public transport with the locals and visiting local grocery stores really can give a quick answer to the "Would I feel confortable living around these people here?" question. I decided to not move to Germany after visiting. I even visisted it 2 times, different provinces and it's just not the kind of culture I feel good in. On the other hand going to Slovakia always feels like going to my home away from home, from my very first trip there.
Anyway, there are organized group trips to Zagreb as a base where they visit Plitvice lakes as an all-day trip one day and then the next day they visit the Postojna cave in Slovenia. Some all add in Ljubljana to the itinerary. In my country tour agencies offer you the possibility to opt out of these day tours. So I can skip the Plitvice lakes and use that day for my own, self-organized trip by train to Trieste. So I wonder if several hours would be enough for Trieste? I'd like to see the old town, enter a grocery store, use the public transport (not just buses, but the interesting Trieste–Opicina tramway is finally up and running!).
What would you say about Trieste as a day trip from Zagreb? A train or Flixbus would be better between the two cities?