Split to Rovinj
 

Hi- Would like to plan approx 7 days in Croatia. Renting a car in Split and heading to Zader, Plitvice Lakes Natl Park and Rovinj. Will 7 days be enough? Are there better towns to stay in?
Which of these towns needs extra days? Also, anyone have info on the ferry from Rovinj to Venice? If I am going in May, how far ahead must I book the ferry? Thanks!

A few more days would be better but your plan is ok, use Rovinj as a base for a few days to see Istria.
The ferry leaves for Venice at 7am on Wednesday, Friday and Saturday during May, cost is about €60 with no need to book in advance.

Thank you LittleVicky. If I have more time, how would you break it up?

It just depends on what you want to do and see and how you like to travel (i.e., relax or sightsee). Have you plugged the drive times into google maps or viamichelin?
I like to stay overnight at Plitvice, but you could stop there in between Split and Zadar.

I agree that your plan could work quite nicely. It all depends on your interests and preferences.