We’ll be travelling to Croatia with our 11yo son the second week of July, flying into and out of Zagreb.
Planning to rent a car for the trip.
Our main interests would be: sightseeing, beaches, culture, food, children related activities.
We are thinking about visiting Zagreb, Plitvice Lakes National Park, Istria and the islands, as we don’t want to rush. Split will be the Southernmost point to explore this time.

Please advise on the itinerary, accommodation (Airbnb), local food restaurants and attractions.

Thanks in advance!

Exactly how many days do you have for this trip? If I am taking a week literally (so, 7 days), figuring in your flights to/from home (or where you're coming from) - you really only have 5 days on the ground. Plus travel from city to city and BACK to Zagreb - your time is very limited.

Have you already booked flights?

If you have not, consider flying into Trieste or Pula - that gets you into Istria quicker. And out of Zagreb. With that having been said, you barely have enough time to explore Istria let alone going to Plitvice AND exploring Zagreb. You have no time for Split on this trip for the amount of time you have. You could fly into Zagreb and out of Split, but you will also have to get up to Istria too.

In short, please advise how many days on the ground you would have.

I think you could benefit from doing research using guidebooks. This will help you determine what is available on each locale, especially Istria, which will help you plan what you want to see and how long you will need.

Also, think in terms of Nights rather than days. 3 nights in one town equals 2 full days due to time in transit, checking in/out hotels or airbnb's, etc.

Can't help you plan where you should sleep without a clear itinerary. For now, check out Airbnb yourself for their reviews. Booking.com is also a good resource.

Keep in mind the July is high season and expect crowds, especially in Plitvice. You may also have cruise ship traffic to contend with in Istria as well.

Travel_Nerd is, IMO, absolutely correct about your time and I agree that a guidebook would serve you well. (I found the Rough Guide to Croatia the best of the available guidebooks.) And even if you actually have 7 days on the ground, I think you are being overly ambitious; YMMV. Zagreb can easily take several days (depending on your interests). With the crowds in the Plitvice Lakes, you should plan on the afternoon of one day and the morning of the next. Istria easily takes 4 or 5 days -- even if not on a relaxed pace. The islands could take weeks! Split can easily take days.

Good luck!

Thanks everybody for your replies.

We will have 7 full days on the ground, flights have been booked into and out of Zagreb.
Most probably we'll save Dalmatia for another trip.
We like hiking, nature views. This time we are not that much into cycling, aquariums, dolphin tours, aqua parks. We'd love to do a boat tour as well.
My son loves mysteries, medieval festivals, pirate ships, zip lining and the outdoors classic music concerts sound amazing!
Hope to find some sandy beaches, I know they're quite rare.

Thank you very much for your time and effort.

I can't help with the beaches, but for hiking, nature, and views, the Plitvice Lakes National Park makes a lot of sense, and maybe the Krka National Park, too. From what you say of your interests, I'm not sure Zagreb will satisfy you, but if you are flying out of Zagreb, I would think you would want to be there the night before your flight.

If you are crossing multiple time zones before your arrival in Zagreb, please wait at least a day before renting a car! Although many people are not aware of it, there is mounting evidence that driving with jet lag is just as dangerous -- to yourself and others -- as driving drunk, and nothing you can do will prevent the microsleeps (which you might not even notice) that are the apparent culprit. Seriously -- NOT a good idea, no matter whether you sleep on the flight and no matter your prior experience! And please be patient with me if you think I’m being overly adamant, but I know too many people who have died or been seriously injured in accidents to which jet lag was a contributing factor, and I know too many people who will spend the rest of their lives dealing with the knowledge that they were responsible for accidents that resulted in multiple deaths, including those of children, all because they were too confident in their ability to drive safely with jet lag. The issue has become a bit of a cause célèbre for me.

Depending on when your flight lands in Zagreb, you might want to take public transportation to Varazdin, which has an interesting castle (Stari Grad) and a wonderful museum of entomology, which your son might enjoy. I visited both as an adult and found them quite interesting, and saw many young people who looked like they were having a great time.