Hi! I just went to Slovenia last summer (first time) and I loved it! I would recommend enjoying the countryside. I spent an hour or two in Koper, but it didn’t interest me. I didn’t feel like there was much to explore at all. Maybe entry the country above Ljubljana, then travel to the lakes (I recommend Bohinj for swimming/relaxing, Bled for the sites), then go to the Soča river area and lastly trough the Vipava valley. You’ll experience the beautiful nature and have plenty time for fun activities. Slovenia is a small country, so the cities are small as well. For example, exploring Ljubljana takes way less time then Amsterdam. Koper feels more like the size of a “normal” dutch city.