Day trips from Ljubljana

When we went to Ljubljana we did two day trips:

1- Skocjan caves- you can get here by train and then a bit of a walk. Very impressive cave system- you enter through a small tunnel and then it opens up into a huge expanse. We really enjoyed our visit.

2- Lake Bled- you can also get here on the train. We really enjoyed the walk circumnavigating the lake and the local Bled cream cake. There's also an island in the middle you can visit during the high season, but we were there in winter so the boats weren't operating.

There are many others but I think these are two of the most common ones. Good luck with your planning.

Plitvice is way too far unfortunately, but you’ve nice options around Ljubljana. Bled has regular bus and train service, but bus is generally preferred as it drops you right in town near the lake. A day was plenty for me (and I would totally skip the boat ride out to the island.)

While it’s conceivable you could figure out some public transportation to the caves, it would be much easier to take a day trip tour. There’s a few well regarded companies listed in the guidebook. I did not end up using any, but the folks at Slovenia Explorer were very helpful and responsive. They have half-day trips to Postojna, as well as Bled. Full day trips go to Skopjan and Piran. They had an interesting selection of itineraries in addition to those too.

In case awrzesinski's post is not totally clear, Postojna Cave is another cave system,
a bit closer to Ljubljana, and reachable by public bus (it drops you off in the parking lot).

Predjama Castle is nearby and you can buy a combined ticket for both locations. There
is a shuttle bus between the cave and castle as well.

Obviously a guided tour makes things easy, but the public bus is cheap and not that hard
to figure out.

Skocjan Cave is more dramatic, but you are in the dark much of the time. Postojna Cave
is a bit more well-lit and the tourist infrastructure is much better. Both are worth considering.