Three days with Ljubljana as a base

I suggest spending a that day at Skocjan Caves and Predjama Castle, unless there's something you really want to see in Piran. IMO, local festivals are usually fantastic ways to explore an area. Maybe you can find YouTube videos from previous years to see if you'd enjoy it. You could rent a car or find a driver to walk at Vintgar Gorge (if you're not doing that when you go to Lake Bled) or Mostnica Gorge/Voje Waterfall. Or rent a car or hire a driver to spend a day in the Logar Valley. IMO, these other places are more beautiful than Lake Bled.

There are a number of small group tour operators running out of Ljubljana with a lot of options. I recall the Skocjan-Piran itinerary as a common one. While I didn’t end up using them, Slovenia Explorer were very helpful and communicative.

Don’t feel compelled to fill in those other two days. Ljubljana is a great place to explore with some fine museums and sights that I’m sure a day tour won’t cover. But its rapidly expanding popularity probably has less to do with its museums and more to do with its beauty and vibes. Ljubljana has its own brand of La dolce vita, especially when the weather is nice. I would kick around in Ljubljana for a couple days easy.

I think Škocjan Caves + Piran would be a good combo and would be of higher interest to me than Škocjan Caves + Predjama Castle. Škocjan Caves is amazing; Piran is quite lovely. Slovenia Explorer has historically offered excellent small group tours (no more than 8 per group) at a reasonable cost. It looks like they are somehow affiliated (or owned by?) Gray Line now, which is something new, so I'm not sure how that impacts how the company is run. Slovenia Explorer has a small group day trip from Ljubljana that includes Škocjan and Piran: https://www.slovenia-explorer.com/scheduled-day-trips/skocjan-caves-piran. They can do a private tour, too, but they tend to be kind of pricey for these. I used the company for a private tour of Idrija and Franja Partisan Hospital in 2019. Excellent guide. Exact tour I wanted (+ the guide threw in some time at Škofja Loka, which was great). Pricey! But worth it to me.

Another option for a private tour that I think would result in a better price is Mateja. She is a very professional guide with whom I did a couple of days of touring in Ljubljana. She arranged a reasonably-priced transfer to Piran for me with stops at Predjama Castle (viewed outside only), Park of Military History Pivka (military museum), and Škocjan Caves, but she of course could create whatever tour you want. https://sloveniaguides.si/en/

As an aside, if you want a really unique "souvenir" of your trip, check out Nika's Tiny House while in Piran. She creates very cool stuff/art from driftwood. https://nikastinyhouse.si/

As for the music festival, here is the link if you haven't found it: https://drugagodba.si/en/. It looks like it is a modern spin on international music. It is in its 42nd year and appears to have a good reputation. Looks very cool to me.

Enjoy your trip! Slovenia is one of my very favorite countries!

Here is the site for the music festival. You can listen to most of the groups on you-tube: https://drugagodba.si/en/

I will be there May 3-12.

Spending a few days in Piran to chill out, then onto Ljubljana with a day tour to Lake Bled. Taking different morning and afternoon Guru walking tours. Found them to be perfect for me.