Plitvice harbors a stunning natural park featuring a lake system that has created dozens of waterfalls and ponds. I strongly urge everyone to visit, the park is easily accessible and the trails are very easy. There's even a boat/bus system to take you around if you don't want to walk. I really want to revisit this place in winter. You should also check out this amazing picture.
And some bonus pictures of a nearby cave we visited.