Plitvice is a must do, but they have an online ticketing system now so book in advance. I may be in the minority, but I didn't love Dubrovnik. I would suggest Split and touring the islands. Hvar (Stari Grad is much more laid back than Hvar town), Korkula is supposed to be great, and has the walls like Dubrovnik without the lines, but I have yet to visit there. I loved Zadar also