I visited this great natural WHS in April 2014. I spent 3 days here and it was worth it to have more time to visit at leisure. The entrance to the national park costs 14.50e (if you stay at one of the hotels inside the park this entrance fee will be valid for all your stay) allowing you to use the boat trips from one side to the other of the largest lake or else use the electic vehicles that take you from one entrance to the other. I didn't use the latter since I had ample time and decided to hike instead. The trails are really pretty and allow you to observe the abundant flora and fauna of the park. There are ladders that are placed on puddles so that you can easily walk the trail without wetting your shoes. However, good hiking shoes would be advisable. I visited both the upper lakes and the lower lakes and both are extraordinary. However, if I had to choose one, I'd go for the lower lakes with the big waterfall (Veliki Slap) and the "walk on water trails". The highlights of my visit were the panoramic views from above the Sulpjara Cave and just opposite the big waterfall with all the rainbows. Plitvice Lakes National Park is definitely one of the best natural WHS I visited in Europe.