We arrive in Zagreb on July 5. Spending 3 nights in Zagreb but open to side trips out of Zagreb because we know we probably don't need 3 days there. 3 night accomodations were part of the cheap deal we got. What side trips would you suggest?
Heading to coast early Sunday July 8 for one week. Must return to Zagreb on Sat 7/14 for flight departing at 1300. Yes, I know we should have planned open jaw, but we saved 1K by flying into and out of Zagreb.
Don't know if we should drive, fly, bus or train back and forth between Zagreb and the Coast.
Don't know where we should stay on Coast. Thinking Split, so we can vector around to islands but open to suggestions. We like to explore historical towns, eat new food and meet locals. Love beaches. We are traveling with our 24 year old daughter, who will probably want to go on a pub crawl or two.

For Zagreb, go to Samobor or Varazdin, as a day trip. Zagreb has great museums.

It depends. I see no reason to drive unless your plant to stop at Plitvice National Park. For Split, bus is simple. I suggest reading up more about the islands so you can choose 1-2 that appeal to you. To me it is preferable to stay on an island versus lots of day trips (especially in July).
Your daughter may enjoy Hvar.

I wish I had had a car my entire trip. You won't need one in Split or Zagreb. Zagreb has a superb cathedral. Your daughter should like the nightlife there. There is a pedestrian only area where there are many, many cafes. I don't know how you even chose.