I am planning a beginning of October trip to Croatia. Was thinking the following:
We are active and in our in our 70's- more into off the beaten track things- meeting locals - not too many Churches- perhaps some a few Synagogues -good food and wine , local markets and art
2 nights Dubrovnik
2 nights Split
2 nights Zadar
2/3 nights Rovinq
2 nights Zagreb

Please let me know your thoughts? Do I book any tours ahead of time or do I do that when I get there?

I can certainly understand why you want to visit these places, but your plan is, IMO, extremely ambitious. Most of your transitions will take at least half a day, so you'll generally have only 1 day and parts of a day or two in any of these locations. And with Dubrovnik, you might lose time to jet lag. With some of these cities (e.g., Zadar), that might be enough for you; with others (e.g., Split), you might end up wishing you had more time.

I can't say whether this trip will meet your interests, but I can say that you are not going off the beaten track -- quite in contrast, you are targeting cities with lots of tourists. (Be sure to check cruise schedules -- some of these places become absolutely overrun when there are cruise ships in port.) I don't know how you plan to "meet locals." I could easily be wrong, but I'm not sure there are synagogues in Zadar or Rovinj. If it helps, Split has a wonderful market and Dubrovnik, Split, and Zagreb have some interesting art. I felt no need to take any tours in Croatia. YMMV.

Honestly, I think you are planning a very rushed experience, and one that will skip some of Croatia's most magnificent treasures (e.g., the Plitvice Lakes). If you have 12 or 13 nights, you might do better to plan to visit a smaller portion of this huge and wonderfully diverse country.

I hope that helps!

I agree with kja. Your plan currently has you spending more time getting to each location than actually being there. And I'd say all of your itinerary are crazy busy with tourists.

How are you planning on traveling? If renting a car, you should not drive 24-36 upon arrival if you're crossing multiple times zones (i.e. from North America, Asia, or Oceania).

I know Dubrovnik and Split have synagogues, maybe so does Zagreb. But I'd be honest, Croatia has not had much of a Jewish population. Sarajevo (Bosnia) has an interesting history of Jews, but it was all but decimated in WW2 by the Ustache regime - which probably also happened in other areas in the region too. Not to dissuade but rather to inform, if this portion of your trip is important to you, I'd suggest retooling your itinerary to focus on towns/areas where you would have more success with your interests, if that is the case.

I would spend more time in Dalmacia and skip Istria,

If you have 11 nights in Croatia, I think you should only visit 2, maybe 3 cities. My picks would be Dubrovnik, Split and Zagreb. There's nothing wrong with the other places, but you don't have time to visit all 5 of the cities. Trim back your plan and we will help if we can.