hi everyone, my husband and i are a planning a holiday to croatia. we're having a few arguments on how long and when to go. i, of course want 4 weeks my husband 3. i want to go middle of may to middle of june, he would like june or july. we're renting a car, and most likely flying into zagreb. i'll tell you our hoped for itinerary. fly into zagreb, stay maybe a day, then on to slovenia for 4 days. back to the istrian region for 4 days or so. we're thinking of driving the coastal roads as much as possible. we're planning to visit a few hilltop towns and then to rovinj. from there, maybe opatija and krk. i may be off geographically as my guidebooks are in the mail. from there where would you go? we'll go to plitvice lakes on our way back to zagreb at the end of our trip. we'll head to hvar, korcula and mljet. would you reccommend these as must see's or just one or two of them. then we'll go to dubrovnik for a few days with a side trip to mostar. after that a few days in montenegro, and then drive back up to plitvice lakes and back to zagreb. what i need to know is can this kind of trip be done in 3 weeks or would 4 make it more doable. what town or city would you spend most time in? any hidden gems that i don't know about or haven't mentioned. we're open to most everything. we're in our mid forties, not big partiers, so nightlife isn't a big draw for us. we love history, exploring out of the way places as we have a car, walks and swimming. my husband has stated he must swim in the adriatic, a lifelong quest of his. is this possible last 2 weeks in may, or would june really be better? this is just the beginning of my questions, just wait till my guidebooks arrive. by the way what guidebooks would you reccommend? hope i haven't exhausted everyone by my long post, but i use thorntree whenever we travel and the information people provide has always made our trips special. thank you to all who help!!
you've done your homework, I see you are ready to go. Good itinerary, like it. I'm from Split and would recommend it, not only because of the 1700 years old palace, but many other attractions, beach in the middle of the city and people, who might seems rude and loud, but when you get to know them it's a whole new story.
Hvar is party island, but worth a day visit. While in Hvar you have to go see Pakleni otoci which are little islands just off town of Hvar. The most famous beach is on islad Brac and it's called Zlatni Rat in the city of Bol, that's wort spending one day, your husband would enjoy swimming there. Korcula is amazing, Mljet is a national park, my favorite is Vis, and I would suggest you rent a boat with skipper for a week (or 5 days) and see those island that way. That way you'll see hidden beaches with crystal clear water that you can't access otherwise.
3 weeks could be enough, but 4 weeks is better option, 4 weeks give you time to spend at one place you get to like and to meet people which for my traveler's preferences is something I enjoy the most when going abroad. May and June is better option for my taste, water might be little bit colder, but I don't mind that. July could be too crowded, meaning, you'll need to pre book almost everything and beaches will be crowded. Also it gets too hot, so in July and August it's too hot to even walk to the beach.

Three weeks is okay, but it depends on how long you wanna spend in Montenegro, as it also has quite a lot of offer, nect to Kotor & Budva. There is for example the city of Cetinje, former capital of the country, and the Durmitor National Park, which is a perfect hiking place.
About your itinerary, I think it is quite okay to go south down along the Adriatic Coast and Dalmatia until Montenegro, but maybe, to avoid doing twice (almost) the same, it would be better to come back via Bosnia & Herzegovina, which is a really nice country, althoug much less touristy, but very friendly people, and nice and unspoilt nature. Don't miss Plitvice of course !! Other things you shouldn't miss are: Dubrovnik, the Karst region and the Julian Alps in Slovenia, Istria, Mljet island, Sarajevo, ...
But if you will go through Bosnia eventually, just remember that the roads are much less developed, so it will take you much more time to travel.
Enjoy anyway.