Trieste touring

Well, I think it's a mistake to try to see the town and even one neighboring country when you'll have only one day. Trieste surely has enough to keep you occupied for a day, and you could see it on your own without spending a lot of money for a private tour. Or you could see a bit of Slovenia or Croatia. Why try to do both when it would just mean more time sitting in a car?

Or you could hop on an Italian train and go to Venice for the day. That would also be inexpensive.

I have no experience with tours from cruise ports, but you might check to see what options MyDayTrip recommends. That company connects travelers (not just cruisers) with transportation providers. They market to people traveling from place to place who want to make sightseeing stops along the way, but I don't see why they wouldn't be willing to so a loop trip out of Trieste. Try the "Request a custom route" link.

Or Google something like "port tours Trieste".

Ljubljana is worth more than a day and is something over 1 hour from Trieste; that would be one possible destination. If you just wanted a quick walk around the lovely city center (not what I would do, but it's your trip), you could also see Lake Bled.

Alternatively, you could get a look at the Istria peninsula. I'd guess most transportation providers have some sort of canned itinerary for that area. I'd hope to see at least Rovinj and one of the medieval inland villages; those are both in Croatian Istria. The Istrian scenery is nice, and there are many other worthwhile stops, including colorful Porec, tiny Vrsar and Pula (with some Roman ruins). I think Koper is the primary sightseeing target in Slovenian Istria.

I agree that if you only have one day you should focus on Trieste itself. Going to Slovenia and Croatia aswell would be too rushed in my opinion.

Trieste is a charming and interesting city — we can’t wait to go back. Take the bus out to Miramare Castle. We were there on a beautiful day and it was one of the most gorgeous places I’ve been.

Enjoy!

I agree you don't have enough time to go to neighboring countries. We arrived in Trieste with a driver from Croatia and spent 2 days before going on to Venice. You can take the train to Miramare Castel just outside the town and it is fabulous.