Hi. A group of 4 of us will be spending 10 nights in Croatia in Sept '13. Flying into Dubrovnik, exiting from Zagreb. Have several areas/destinations of interest, so realize we need to pare it down a bit. They include: Dubrovnik, Kotor Bay (Mont.), Mostar (B-H), Hvar, Trogir, Zadar, Rijeka, Zadar, Plitvice Lakes, and Lake Bled and/or Ljubluana (or other recommended destination) (SLO). Open to any form of transportation except hitchhike. Help?! and Thanks.
Buses work great and run frequently up and down the coast from Dubrovnik to Split; you just need to find the main bus stations at each location. For Montanegro, you can easily rent a car and tour around. For Hvar and islands, there are ferries from Split. To Mostar, you can also take a bus.
(I have not been to Slovenia or north of Split, so cannot comment on other locations).
With 4, I'd get a car. Roads in Croatia are just fine. I will say that locals who drive on the roads from Zagreb to PL are, in two occasions, flat-out insane. However, cars are very convenient, and with 4, I'm sure you'd save a lot of money, and also aggravation. Driving in Europe is not difficult, and is easier in some areas than public transit.
You're going to need to do more than "pare it down a bit". First, start being honest with yourself: never mind how many "nights" you will have, how many FULL days will you have, not including your arrival or departure days? Regarding the places you list above... You have Zadar listed twice. I was there one year ago, and given the other places nearby, I'd skip it. We actually really liked Sibenik, not too far from there (and overlooked by Rick's book). I can't think of any good reason to include Rejika (looked like an ugly industrial city as we drove by). Places on your list we liked especially: Dubrovnik (1 day was enough); Kotor (full day trip from Dubrovnik); Trogir; Plitvice; Bled. We didn't care much for Hvar, preferred Korcula much more. You will want to at least pause briefly for a look at Split (you will be going right by it), although we were not charmed by the place as others obviously are (1/2 day there would have been enough for us). You will need a car for some of this (Plitvice, Bled, Kotor) to do it efficiently; other places a car would be useless or an expensive headache (Dubrovnik, any island). We didn't go to Mostar or Zagreb so can't comment on those...but from what I've heard about Zagreb, you might consider skipping that entirely if you can (fly out from Ljubljana instead?).
Oops - only meant to list Zadar once. And instead of Rijeka I should have said Rovinj.