If you find yourself in Croatia, make sure you stop by Plitvice National Park. Our visit was one of my favorite stops during this leg of the trip. Paths have been set up so you can easily walk along wooden/dirt trails, taking you through different lakes and passing some gorgeous waterfalls. The water is incredibly clear and you can see various types of fish and aquatic plants from where you stand (no swimming allowed).
A tip: plan your visit to enter the park at 4pm when the entrance fee is cut in half. It is much more pleasant to walk around at this time because most of the tour buses visit in the morning and have exited by the afternoon. We intended to take trail C, but somehow got on trail H and trails E/K at some point. Regardless, everything is marked and we were able to get back on the right trail. Just make sure you know the ferry times - we had just missed the longer ferry, which we wanted to take, by 10 minutes!
Krka National Park is lovely, but couldn't even compare to our visit to Plitvice. It is well maintained with easy-to-follow walking paths, but I personally found Plitvice much more impressive than Krka. Additionally, we went to Krka during the middle of the day, so the paths and lakes were extremely crowded. The only upside is that you can swim in Krka's lakes, but we we were so exhausted navigating through the crowds, we passed on the opportunity.