Looking at the map, the coast isn't that close to Zagreb. Most of the places people go to are further down away from zagreb.
Much of the coast is pretty much same wherever, so I suggest looking into a spot that is easiest logistically.
I found Senj, which also has a fortress nearby, and is a total of 3.5 hrs Plitvice-Senj-Zagreb drive total. Places like Rijeka will be 4.15, and the more common areas like Zadar will be 4.45.
With these times in mind and the fact that youre flying out from zagreb, it may make sense to explore the areas closer to plitvice (beautiful mountains) or do more plitvice