Coastal Croatia is a very popular vacation destination for Europeans. I had difficulty extending my stay in Rovinj (Istria) in 2015. Even though you're traveling in June rather than in July-August, I'd be wary of totally winging it. As soon as you know where you are headed next (even if you don't know the arrival date for sure), check availability on a site like booking.com to be sure you're not going to need to stay somewhere else. Then make a reservation when your arrival date is locked down. My assumption is that islands could be especially challenging. You certainly don't want to arrive on an island, find zilch available, and have to wait to take a ferry back to the mainland.
The internet makes it very, very easy for travelers on the move to remain relatively flexible yet still make reservations 24-48-72 hours in advance. Most of the folks competing with you for rooms will have done that, so if you roll into town without a reservation, you'll be dealing with leftovers or coming up empty.
Croatia has a culture of private rooms in people's houses. You may see signs; I think the term is "sobe" (that's probably plural). Many sobe can be booked at little store-front travel agencies, but I don't know whether they can help you out with places in other towns and cities.
As critical as it is to stay in or near Plitvice National Park the night before your visit, that is not something to leave to chance. With a car you have the advantage of being able to stay somewhere that isn't right on the bus route, but I wouldn't push my luck there.
