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.