If you are going to visit National Park Plitvička jezera (what you must do :)) be sure that you are fit for it. There are a couple of different routs. The shortest is about 2 hours and the longest is about 8 hours. That means a couple of hours of hiking. So be sure to equip yourself with good shoes,comfortable clothes and plenty of drinking water. And enjoy it :)
Fun Alternatives: If you aren't so fit but you don't want to miss this beautiful place there is a way also for you to see it.
Just choose the shortest route and you will find that you can use a tourist boats or tourist buses that are visiting lower lakes. But one part of lane you will need to walk also - so comfortable shoes, plenty of water and go on.