Hi vadu,
Apart of Plitvice, I highly recommend for all who like sea and summer to visit Istria as well as Plitvice.
If you haven't been to Istria, Istria is west and upper part of Croatia that borders Slovenia and 20 km away from Italy. It has beautiful beaches and represents top of Croatian summer attraction. I highly recommend specifically Rovinj, Pula and Porec and eastern part of Istria with magnificent country side.