2 week itinerary, now in September (anything I should know thats different for Sept vs. June?)

While staying in Rovinj you can also see Porec and the tiny coastal-hilltop town of Vrsar. Inland, Groznjan is also attractive.

Zagreb has a very pretty bi-level historic district. There's a funicular that gets you most of the way to the top, but you might opt just to taxi up. I guess there may be buses; I just walked.

I'd urge your group to research the museums in Zagreb before the trip. There are a lot of interesting ones, mostly art-related and some small enough to be seen quite quickly. There might be something that really appeals to at least some of your group. I'm especially fond of the Naïve Art Museum; there aren't many of those around, and Zagreb's is a fine, though small, museum. It's on the (US) second floor, however, and I don't think there's an elevator.

Thank you, I forgot to mention that my aunt's ethnic heritage is Croatian, she's an artist so she would love the art museums. My dad and my uncle are avid cyclists. My uncle is a foodie. They all love coffee. Seriously. Its part of their identity. My mom is a blend of them all. hah.

Yes it sounds like at least one more day in Zagreb might be warranted.
Depending on departure time, it might be prudent to spend the last night in Split.
Check ferry/catamaran schedules to make sure you can stop in Krka and get to Brac in the same day. Also, peruse the schedules for day trips, which are not always convenient from an island base (routes often go through Split). Operators are Jadrolinija and Krilo.
Sounds lovely, enjoy!

You will be so close to Dubrovnik...if you haven't been, do take a look: I think it far surpasses anything else in the region. (See my recent trip report!).

Is it feasible to do a daytrip to Drubovnik from Brac or maybe even Hvar? Haven't decided which island to make a base.

Nevermind I answered my own question- looks like its over 4 hours away, better do add it on the itinerary. Maybe do 3 days in Drubovnik at the end and fly out from there.

Hi - I have traveled in Croatia and Slovenia several times (4), have used Rick Steve’s Book like a treasured friend each time. I am of your family’s age, so I can relate well to their interests ( museums, adventure and food)..
First, be aware that flying into the Capital is different than flying out of Dubrovnik - all transportation here must be well planned: car rental and return, limited use of bus, very few train lines, excellent toll roads and ask rental car company how to use them, strict ferry schedules ( and know the difference between fast Cat and the car carrying ferry.)
You can plan to either have your trip begin and end from the same location or do a logical one direction trip ie north to south seems to be your preference. Personally, I would rent car in Zagreb and drop it at the Split airport after leaving luggage and a few travelers in Split, returning to accommodation by taxi. I would then travel on the Fast Catamaran through the islands to Dubrovnik. I would taxi to Dubrovnik gate and find accommodations inside of the old city.
To return to Split I would hire a driver, take the coastal road, make stops along the way to spend 1 night in Split and fly out of Split! The car will not be as important as you might think on the islands.
The capital is a large city- interesting and I’m sure some people could stay a week, but you dont really have much time and want to see quite a few things.
Straight to Rovinj, stay in old town, spend the money to park your car close for day trips:
I found a nice place on the water with two bedrooms (4 people) for a reasonable price. Restaurants galore, saw a Miro exhibition there in a small art musings - Rovinj has a charm that is unsurpassed and at one time was Ricks favorite town. I adore old town.

Day trips include (1) the hill towns (most see Motovun, I’ve walked the walled pathway (nice) but I have stayed in the small artist town in Groznjan ( lovely);
(2) south to Pula, the 6th most complete Roman Amphetheater in the World; Tito’s home the island of Brijuni; (3) Drive to the Lakes maybe a 5 hour drive, but longer on coast to see Opatija (nice), spend at least one night near P.Park. Your plan is good, but don’t take the wrong walk like I did and made my 90 year old husband exhausted - be sure to go in the direction that will be mostly downhill; Skip Other K.Park (4) Drive to Zadar it has historic old town, pick accommodation carefully. There is one of the nicest small Archeology Museums I have seen - worth the stop! (5) Split at least 2 full days, do not miss the famous sculpture’s US citizen who did the Two Indians on the ends of Bridge in Chicago and La Pieta at U Notre Dame South Bend : Ivan Mestrovic Gallery; return the car - there imay be return offic for your car in Split too. (6) Begin journey by fast Cat: 1 night in Bol, a couple in Hvar, a few more in Korcula ( which I like best) - remember the sword dance Thursday night, then off to Dubrovnik to stay inside the walls... I stay close to the upper gate. (7) Return to Split by hire car/van along coast to spend last night in Split - Fly out the next day. I loosely counted days and came up with about 16... it is a great trip and I don’t know your flexibility in length. Enjoy Croatia it is wonderful!
Four trips, longest 1 month. Susan

Oh, weigh your luggage and try to stay under 40 pounds - you will do yourself a favor. I carry no more than 32 pounds and a back pack - for months!

I will add only one thing - Dubrovnik is hilly. Depending on the level of "very active", they may or may not be ready for Dubrovnik. At our first sobe, we had 450 steps to the Strada, the main street. We took it slow (64 and 67 at the time). If you get a place in the old city, you won't have that problem, but it will be more expensive.

If you want truffles, go in the fall.

I spend every September/October in Croatia, mostly Rovinj, Zagreb and Istria.
Totally love the Naive Art museum in Zagreb mentioned below, visit it annually. Good postcards. Upper town an easy walk up any one of several hidden steps, cobbled streets, etc.. Zagreb has transformed into a vibrant mix of restored old and lively new. Do stop on Tkalchicheva or the main square for coffee and just watch people. University town, lots happening. You may want to pick up your car rental as you leave, to reduce driving in a hectic city with trams coming in all directions. Walking is best in the core.

Paying to park in Rovinj is a hassle, but it allows you to stay downtown at any one of a zillion rooms listed on booking.com. This in turn allows you to sleep in the old town and wander it before and after the bus groups. Take good rolling luggage. I am a big fan of Eagle Creek's Load Warrior (after years of using Rick's Eagle Creek bags) with its two quiet wheels.

Istria is best in September, especially Grozhnjan and Oportalj, and Grasisce, as well as busy Motovun. Rovinj tourist board website lists dates of local events like mushroom and truffle days. Google maps does surprisingly well in the hill towns.