one day in Ljubljana

Dig up a list of museums (unless it's a Monday) and see if any of them interest you. There aren't any world-class museums in Ljubljana, but there are some decent ones. I enjoyed a tour of the Jože Pučnik house (guided but only about 30 minutes) - he was a key architect who designed many prominent buildings and bridges you'll admire in Ljubljana.

Otherwise, walk around half the day and explore. Ljubljana is a charming, vibrant town, compact and great for walking. Hike up to the castle for the great view down (inside the castle is nothing amazing as I recall, but I've seen enough of them). Walk over to Tivoli Park. You can even take a walking tour.

Slight correction:

Jože Pučnik (1932-2003) was a Slovenian politician and one of the fathers of Slovenian independence from Soviet influence. The Ljubljana airport is named for him. I am not aware that there is a house of his in Ljubljana that you can tour.

Jože Plečnik (1872-1957) was the architect responsible for many of the buildings and bridges you will see in Ljubljana. You can indeed visit his house (https://www.visitljubljana.com/en/visitors/things-to-do/sightseeing/plecnik-house/).

Andrew's suggestions are spot on. You might find it interesting to join the free walking tour (https://ljubljanafreetour.com/). I thought it was excellent.