Layer it
by CCW
If you plan to travel by train or bus, bring a suitcase or duffel bag rather than a backpack: oftentimes you'll have to pull your bag out of a tight compartment in a hurry, and straps will inevitably get caught. (Remove your shoulder strap from your bag before stowing it, for the same reason.)
Fall can be rainy, so bring a waterproof jacket and at least one pair of water-resistant shoes. The climate is generally mild, but it varies by region. The Julian Alps have a mountain climate: it almost always gets cold at night, so don't go out for the evening without a jacket. The coastal climate is more Mediterranean (but not as balmy as, say, Greece), and Ljubljana and vicinity are moderate. I advise the layered look: for a fall trip, I brought a waterproof jacket, a cotton knit jacket, and a fleece vest, and on the coolest nights I wore them all!
Oddly, in many European countries, vitamin...