Having just come from Ljubljana (where the weather was pretty lousy), I would offer
that 5 nights is a lot if you expect to have lots of interesting things to see. You can
probably spend a day at Postojna Cave and the castle, but I think you'd get bored
3 days in.
On the other hand, if you want to sit at a cafe on the river and watch the world go
by, you can do that for months.
You could take 1 or 2 of those 5 nights and go to Bled/drive the Julian Alps and still
have time to enjoy Ljubljana.
Another thought, you might want to consider 2 nights at Plitvice depending on what/
how much you think you're going to do there. You definitely want to get an early start,
so if you get there midday from Vis and plan to spend the night after the hike, you
won't see as much as you would probably like. But, if your night is before the park,
that might work. Just keep in mind the drive out is not all on a 4-lane highway, you
will prob be tired after the hike, and you need to get to the rental car dropoff location
before they close or know you can drop the keys off in the office mail slot.
Plus, assuming you have a car, a must is to drive over to the scenic overlooks on the
side of the canyon away from the visitor centers. The paths are over by the Villa Nena
B&B, unmarked, but easy to find.