Koper, Adriatic cruise stop, day trip?

I haven't actually been to Koper, but I know of its reputation as an "industrial town." There is no train to take to Trieste, but there are buses. Look also into GoOpti for shared transfers in a van or private transfers; the shared transfers aren't too expensive if booked in advance. Perhaps you could take GoOpti one way and the bus back from Trieste if you can plan it to that degree, split the difference.

Driving in Slovenia is easy. You could probably just rent a car for the day in Koper and drive wherever. Piran is a lovely little town that I HAVE been to and very close to Koper; you wouldn't need to rent a car to get there (take a bus or a taxi), but you would if you want to combine it with something else, a cave or something.

Lake Bled is lovely and probably an easy drive. Consider detouring on the way via the town of Skofja Loka, then do the scenic drive from there through the mountains via Jamnik and Kropa down into Bled. There isn't a ton to do at Lake Bled other than admire the lake, take a pletna boat out to the island and its church (takes more time than you'd guess), or go to the castle above the lake. Some people hike nearby Vintgar Gorge (takes about an hour, you can drive to the entrance). But...a drive to Lake Bled with a stop in Skofja Loka and on to Bled for a few hours then take the highways back to Koper would make for a nice day and not too stressful, assuming you plan plenty of time to get back to Koper.

Doing Piran + a cave would probably be less risky, keeping you closer to Koper. I suppose something could happen up at Bled, your car breaks down or you get into a traffic jam or something - but how likely?

Trieste by bus or transfer is perhaps the easiest. Personally I wasn't a fan of Trieste - seemed like a big noisy Italian city with some charm here and there and some unique history. Little Piran near Koper is much less significant than Trieste but much more charming in my opinion.

I love Ljubljana, and if you like cities, I would highly recommend it too. But it might be a big push to try to see both that and Lake Bled in a single day. There is a train from Koper to Ljubljana (check - it may still be partly replaced by a bus due to track construction), but it is not a fast train - driving would be the practical way to get to Ljubljana with the very limited time you have.

There is an Izola-based couple who offers private tours along the coast and throughout Slovenia (as well as the northern coast of Croatia). I'm staying in Piran for 4 nights in September and have not yet done a tour with them, but I am going to spend a day touring Piran, hiking along a former train line, biking along the coast, and visiting a few other sites with Demian. Thus far, I have found them great to work with and full of ideas for spending time on the coast. Price is quite reasonable for what I am doing.

Having made multiple trips to Slovenia with young children, I can recommend Postonja Cave (Skocjan would be impossible for a 4 year old) and a few hours in Piran eating ice cream. The mountains and Bled and too far - we go slow through those mountains as my kids keep throwing up.. Alternatively, spend the day in Ljubljana. Trieste - meh. Keep in mind you’ll travel half as fast with a 4 year old.

Thank you Andrew H, Dave and Emily for the great advice!

Now you have me leaning towards Piran and maybe a cave. We do try to not overdo it when traveling with our young son. As much as I would LOVE to see Bled and Ljubljana, my husband thinks they are best saved for a future land trip to Slovenia and Croatia. But as it’s our only stop in Slovenia, I do want to see some part of Slovenia other than Koper, so I think Piran is the easiest and best bet!

Also, Lipica is close if you are into horses.