4 Night Trip Itinerary in Slovenia?

I would recomend option no3. Bohinj is a very nice lake and not so far from Ljubljana. Now in this time have perfect nature and weather.
If you have time you can visit Istria which is close to Slovenia where are you staying.
I would recomend to visit some seacoast towns where you can enjoy beautifull sunsets
https://vm.tiktok.com/ZNdBq8VGK/

Or walk thru some nature paths we can be found on more central parts
https://vm.tiktok.com/ZNdBVEMJt/

Thank you for recommending option #3. Unfortunately, I will have only 1 free day after the Zagreb conference and am thinking about a day trip to Plitvice Lakes (taking a bus there and back). On a future trip, I hope to visit Istria (especially Rovinj) with my wife. But this is a solo trip due to the Zagreb conference. Will have limited time for travel exploration (to gather ideas for a future trip).
If it rains heavily in Lake Bohinj in early September 2025, is there a nearby museum I should visit?

Given the time you have, I think I would minimize driving and go with option #3.

As for weather, I went to Slovenia for 25 days in 2019. I was there for the last few days of August and 20+ days in September. One rainy day with sprinkles another day. I like weatherspark for weather averages by month. Here is Sept in Ljubljana: https://weatherspark.com/m/77419/9/Average-Weather-in-September-in-Ljubljana-Slovenia.

Bohinj and nearby Bled are primarily outdoor areas. I guess you could try to go to Bled Castle if there is rain, but I am unaware of other good indoor activities in the area. Kranj and Radovljica are not far away. Kranj has the Gorenjska Museum (a museum about the history/culture of the area) and the Prešeren House with a nice exhibit on Slovenia's national poet. Radovljica has a variety of museums, including a bee museum. I guess there is always the option of trying to extend your time in Ljubljana if the weather forecast is looking bleak.

Regarding the day after the conference, a day trip to Plitvice is, as you will see from many
posts in the subforum, not likely to be all that tranquil an experience. But if you only have
one day, better than not going at all. Just be prepared for the crowds.

I might consider renting a car for the day, as it will greatly reduce the concerns about making
it through the park and getting back to the bus stop in time for the return trip. But if you do
decide on the bus, study the park map ahead of time and figure out your route so you don't
have to backtrack a lot to get back to the bus stop. The park is kind of a figure 8 (kind of),
with 2 entrances, and a boat ride option that can cut off a lot of walking.

Also note it's a nice downhill from the road/parking lots/hotels down to the park itself, which
means it's an uphill at the end when you're probably a little tired and maybe in a hurry.

Obviously check the bus schedules.

Thanks for the helpful feedback. Option #3 is the clear winner. Really appreciate the suggestion to have a rental car for the day trip to Plitvice. There's a free parking spot at the AirBnB in Zagreb. Also, it sounds like a very early start (no later than 8:30am) at the entrance to Plitvice is a good idea. The rental car would also give more flexibility at the end of the visit (vs. rushing to arrive at the bus stop). If I go by rental car, is there a town near Plitvice worth visiting for lunch / coffee on the drive back to Zagreb?

Just had two clarification questions to enhance the trip on days when I will have a rental car.

1. On the drive to Lake Bohinj from Ljubljana, is it worthwhile to consider a slower driving option to visit any of the following towns? Skofja Loka (medieval town), Kropa, or Radovljica (beekeeping museum).

2. On the drive from Plitivice Lakes back to Zagreb, is it worthwhile to stop by the town of Samobor for coffee and to walk around briefly (for 1-2 hours) if I some time before the rental car return by 4pm?

On the drive from Ljubljana to Lake Bohinj, driving through the medieval town of Škofja Loka is a great idea. It’s just 30 minutes from Ljubljana and is basically right on the way.

On the return drive from Plitvice Lakes National Park. the riverside village of RASTOKE is a fantastic place to stop for a meal. Rastoke is on the confluence of two rivers and is called the “Fairytale Village.” Be sure to go into the center of the village ( admission is about €8) for a meal or your coffee to see what put Rastoke on the map.

Thanks for the great suggestion! I plan to stop by Rastoke for a late lunch on the return drive back from Plitvice Lakes National Park to Zagreb. Can't wait to travel in September 2025.

If you are extra lucky, your time in Bohinj will fall on the day of the Cow's Ball. https://www.bohinj.si/en/prireditev/traditional-cows-ball/

Thank you Dave for suggesting the Cow's Ball on Sept. 21, 2025. Unfortunately, I'll miss it by a week. On a separate note, is there a publicly accessible swimming location on Lake Bohinj (or Lake Bled) that folks would recommend for a quick dip if it's a hot day in early September?