Plitvice Lakes
I think there is really no point in describing the beauty of Plitvice Lakes National Park. The fact that it is always at the top of the list of the most beautiful national parks in Europe, without exception, clearly shows what kind of place it is.
This oldest and arguably the most visited national park in Croatia has been on the UNESCO list of world natural and cultural heritage for as long as I have been alive; since 1979. We are both biological phenomena and, of course, we both have our ups and falls.
The national park is open to visitors every day of the year, and let that be your inspiration for regular visits, because the Plitvice Lakes are different in every season. They are my favorite in the winter and I have already chosen a sunny, snowy day twice, picked up the kids from school and went on a trip. Dress very warmly, and good footwear is essential, both in winter and at any other time. In winter, due to snow, part of the park is often impassable for guests.
The tickets are very expensive so it might be a little problematic for the regular visits I suggested, but the national park's birthday is April 8th and tickets are cheaper on those days. Although they can be bought at the entrance, buy them online because that's the only way you're sure you'll really enjoy Plitvice.