You will have a great time exploring Plitvice Lakes National Park.  There are hotels, restaurants and campsites inside the park as well, so you can have a comfortable and relaxed visit, although you may run out of things to do after one or two days.

While in the Park:

Boat Ride

Included in the price of admission to the park you can travel up the lake on electric boats and then hike around the waterfalls. You'll see absolutley beautiful scenery. Note: in the high season it gets very crowded in the afternoon (as the tour buses, etc. come in) so get out early and take the first (8 am) boat.

Hiking

There are miles of hiking trails for you to explore the many species of trees, plants and animals.  

Cycling

National Park had satlleup a 80 km of bike routes,all they can be found on their web site. Combining, biking and hiking around Park gives you a wonderfull expirience cause only then you can say that you visit all National Park .Recomend to take advise from some agency specialized for biking around Lakes.