Croatia 10 days in June

It sound like you are well on your way and have a good idea of where you want to go and what you want to see.

I think I'd start in Zagreb, check out Plitvice and Krka on your way to Split, spend two nights in Hvar, then continue down to Dubrovnik and fly home from there.

That way you wouldn't have to backtrack all the way to Zagreb.

I wouldn't bother doing a day trip to Venice myself. During the day, Venice is crowded with tour groups and feels like Disneyland in many spots, with long lines of English-speaking tourists. But at night when less crowded it's much nicer - in the morning too. I'd guess that many people who do only day trips to Venice wind up hating it.

You could fly into Venice and take a bus or ferry on to Croatia if you wish. I'd skip Zagreb personally if going that way - spend a night or two in Venice then take a bus or ferry to Rovinj, a beautiful town, and head south from there. You can fly from Pula, Croatia down the Zadar, by the way and from there stop at Krka National Park. If renting a car you could pick it up in Rovinj and drive down (allowing you to stop at Plitvice National Park, a slight detour).

The Dalmatian coast is lovely and full of interesting places. Dubrovnik and Split are amazing, but comperatively small, so oyu only really need two days to see what there is to see. The rest of the area has loads of great places, that could be seen in a days time or less and a car would be great.
In Montenegro I would give Budva a miss, the old town is nice, but a miniature version of Dubrovnik and the new town is a horrible tourist hell hole. The bay of Kotor and Kotor town is very beautifull and, in my opinion, much nicer. Also the drive from Kotor to Mt. Lovcen NP is extremely scenic.
Ostrog Monastery is very scenic from the outside, but I found the inside dissapointing, it's very heavily restored and the lines to get in are huge, since it's an Orthodox pilgrimage site. Given the mountains and the road conditions, Ostrog and Kotor/Budva are maybe too much for a day trip from Dubrovnik.
A day trip to Venice is unrealistic, it's too far away and there is already more than enough too see on the Dalmatian coast.

With flying in and out of Zagreb, I'd spend a day or two there, and then fly down to Dubrovnik and work your way back up the coast. This prevents you from back-tracking and the one-way flight on Croatia Air is fairly cheap.

You should go to Plitvice NP in addition to Krka. If I were choosing between the two, I'd go to Plitvice first.

Regarding Montenegro, I really liked Budva, but don't miss Kotor. You will be able to do see both on a day trip from Dubrovnik. You'll pass through Kotor to get to Budva, so it's not out of the way. Also include a quick stop in Perast.

One other place to consider is Mostar, Bosnia. On the one hand it doesn't necessarily "fit" with an Adriatic vacation, but it was one of the most memorable locations of my Croatia trip. It's hard to put into words what I felt standing at one of the epicenters of the Bosnian War, but it is worth your time, especially if you are any student of history.

Overall, it sounds like a doable trip. You just need to make final decisions on your day trips, etc.

Thanks everyone for your input. So we have refined our travel plans somewhat. So far our itinerary is :
Fly into Zagreb. Spend there a day and a half. Fly out to Dubrovnik. Spend there 4 nights. Make a day trip either to Kotor Bay or Mostar.
Rent a car and drive out to Split . Spend 5 night. Day trips to Hvar, Krka, Sibenik, Omis. Zipline at Cetina river. Also would like to hop over to Bol and possibly do a tour of winery. Fly back to Zagreb and catch a flight home. Does this sound reasonable. Was also wondering how to catch a ferry from Dubrovnik to Kolocep, would like to have dinner at Villa Ruza

You will need a holiday to get over this when you get home!

A 2 centre holiday is the most sensible, as local schools will be on their summer holidays by then and some accommodation will have a minimum 3 night booking policy. If you are renting a car from Dubrovnik, ensure that you are permitted to take it through Bosnia. Look into the one way drop fees. (I looked in Portugal yesterday and a one way drop there within the same country added the equivalent of $300 to the hire price.)

Omis isn't worth a day trip and if you use the coast road rather than the motorway, then you will drive through it en route to Split.

Trogir isn't on your list, but should be visited.

Well, so our trip to Croatia is now only a couple of weeks away, We had settled on a couple of days in Zagreb, 4 days in Dubrovnik (with a day trip to Mostar), 4 days in Split with day trips to Hvar and one dedicated to Krka Falls/ Sibenik. We are flying with local airlines from Zagreb to Dubrovnik, and back from Split to Zagreb. But will be renting a car while on the coast. As it is early June , we are a bit concerned about the weather, Not so much about the temperature as we are worried it might rain. Checking lately their weather it seems 5 out of 7 days have been rainy. any suggestions as to what to do on the coast on rainy days.

By the time June gets here, the rainy weather should be past. Typically June, July, and August are quite dry. If you do happen to have a rainy day, use the time at a museum or Diocletian's Palace if in Split. Provided it's not a thunderstorm, you can still explore the Old Towns in the rain. And most of all, use any rain as an excuse to do something you wouldn't normally do -- some of the best adventures are ones you didn't plan.