This may be possibly one of the most beautiful places I have ever seen. Set in the mountains situated between Zadar and Zagreb is Plitvicka Jezera. It is a series of stunning turqouise freshwater lakes and waterfalls. The park is a world heritage site. You walk along the boardwork through stunning scenery but sadly are not allowed to swim. It can be pretty busy with the tourists but it really is a must see.
Recomendation: if you have your own veichle (or you dont mind walk a pair ok Kms), look for a SOBE at both sides of the road near the Park entry. It will be a quiet and beauty place to spend the night, in a house inside the forest, and it will be more cheap than other SOBE in a city. (Nights are a bit cold also in summer, be carefull ;-b).