8 day itinerary

Do not try to see Plitvice on a day-trip from anywhere. It will not be a very good experience, because the park is overrun during day-tripping hours. I arrived mid-morning and stood in the ticket line for about an hour, then I marched through the park in lockstep with far too many other visitors. The only way I recommend Plitvice is if you spend a night in or very near the park so you can go inside as soon as it opens. The last 2 or 3 hours in the late afternoon may also be decent if you can manage to arrive early. That part of the trip will be easier with a car, though there may be a few lodgings outside the park that are within walking distance or provide transportation (perhaps not early enough, though). Check booking.com. There's at least one hotel inside the park which I believe is thought to be not special and a bit expensive, but I have not stayed there.

Rick's itineraries are far too fast for me. Are you sure that layout is even intended to be doable without a rental car?

I would definitely do a guided tour if I go to Plitvice. I would sacrifice flexibility to not wait in lines find parking etc.

I would definitely do a guided tour if I go to Plitvice.

Big mistake, IMHO. I can't imagine any possible reason one would need or want a guided tour for Plitvice. You want to get away from all the other people there, not be tossed in the middle of them.

This corner of Europe is best done with a car - at least portions of it. Some of these places are scattered and are not well (efficiently) connected by public transportation. With 8 days, no way you can pull all of this off via public transport. You need to either cut (a lot) or rent a car (probably more than once - you don't need or want a car for all of this, but in some places, it's critical to making it work).

Plitvice really needs a car to be done in a way that maximizes your enjoyment and doesn't require 3 days. Rent a car, arrive there mid-afternoon, check in to one of the hotels in the park itself (not a great hotel, but its al about location). Go out and enjoy a few hours in the park that afternoon, as the crowds thin out and all the day-trippers (on tours) leave. Get up early, get out and spend the morning enjoying the park before all the day-trippers arrive, When they do, get out and drive away.

3 days in Dubrovnik isn't necessary. One full day there in the city itself is adequate on a fast trip (hope it's a day with minimal cruise ships); use a second day there as a base for a day trip to Kotor (rent a car just for the day, drive there and back, it's easy). I'm not sure Mostar warrants the time/effort required - we skipped that. We did not fall in love with Split, it was completely jammed with cruise ship people and we were happy to leave. Tastes vary but I'd pick Korčula over Hvar any day (unless you want to disco all night). We LOVED Korčula, Hvar not so much.

Be aware that ferry schedules are very, very seasonal - many routes have reduced schedules (or no schedule at all) outside of "high season" (typically July-August, but varies for each ferry line) so it's critical that you confirm that any ferry you plan to use is actually going when you need it (end of May, some of them won't be).

End of May is a good time to go, it's before the worst of the crowds fill up everywhere. But May is coming up fast and booking now, you're a bit late to the game. Giddyup.

Plitvice is about a 2 hour drive from Zadar. It is doable, I took a guided tour from Zagreb ( 2 hr. ) and I enjoyed the fact I didn't have to stand in line for tickets and the guide knew where to go, what directions to take and places with great views that had smaller crowds. You can do it on your own, but I was happy not to map read and just go along with someone that knew the place very well. We also focused on the lower lakes because most of the dramatic falls are there. Whether you choose to do it on your own or take a tour, try to get there somehow. It is a magnificent place to see.

I did a solo 12-day trip to Croatia, Bosnia & Herzegovina, and Slovenia, in June 2017. If you're interested, the (lengthy!) trip report is "Trip Report: Croatia, Bosnia, Slovenia (now all in)" posted on 6/20/18. Sorry I don't know how to post the link. Anyway... I think you can certainly do your trip using public transportation, with possibly a couple of small group day tours. It might look like this: 3 nights Dubrovnik: day one land, day two explore Dubrovnik, day three do a day tour to Korcula. 1 night in Mostar - either take the bus to Mostar, or a day tour and get them to leave you there and not return with the group (see my trip report for specifics). 3 nights in Split (or 2 nights and skip Hvar): day one arrive from Mostar; day two explore Split; day three day trip to Hvar by ferry, or Tragir by bus; 2 nights Plitvice - take the bus from Split, stay close to the park and be at gates when park opens. Finally return to Split by bus to fly home; it's possible that there might be an evening bus that could get you from Plitvice to Split after your day in the park, so that you only spend one night at Plitvice. It's also possible that you might be able to find transportation from Dubrovnik straight on to Plitvice so that you do all of your Split nights together, at the end of the trip. If you do this, it will be a long day by bus, and you will still have to go through Split. Of course, there are multiple day-trip-to-Plitvice tours from Split available; check Viator and Travelocity for possibilities. If you need to cut one night from the above suggested itinerary, I'd cut one night in Split. The website Rome2Rio was very useful in helping me identify specific bus and airline companies that serve these cities.

Here's jmauldinuu's trip report:

https://community.ricksteves.com/travel-forum/trip-reports/trip-report-solo-trip-to-croatia-bosnia-slovenia-part-1-split-croatia

To post a URL as a clickable link, just past it directly from the URL bar on your browser. On older versions of the Forum software you had to do other things to make it clickable, but now, just paste it as a complete URL, and you're set!

Transportations

Montenegro day trips, car or bus.
South islands Korcula and Hvar with speed boat.
Plitvice bus, or car. The train is also possible if your host will come and take you to NP.