Sorry my original post wasn't clear or complete!
We are two couples traveling together who like to stay in apartments instead of hotels. We enjoy almost everything that isn't too strenuous: walks, museums, churches, food, (no extreme hiking!)
We're flying from Tampa, probably into Zagreb and back from Dubrovnik. The flights seem very expensive.
You need to buy some travel books or do some of your our research first. We don’t know what you like, your budget, how long.... put an itinerary together yourself and post it here for thoughts and suggestions. I could easily spend two weeks in Slovenia. I could easily spend two weeks in Istria. I could easily spend two weeks on Brac.
As for flights, use a major search engine like kayak or google flights and see what you come up with.
I agree with Emily that you need to do some general research first - it's like me saying "what should I see in America?" Although these countries are a smaller scale, there is still much to see.
Do you want one base or touring round? What are your interests? How many nights on the ground in Europe?
You will probably have to make a connecting flight - check out sites such as Skyscanner for options. These will vary significantly depending on your exact dates.
Everywhere will still be busy the last week of August with European school holidays, so prices will be higher and there will be less availability this week, as much will already have been booked. Europeans tend to book their holidays much earlier in the year than Americans.
Rick Steeves' latest book on Croatia provides an itinerary and many places to see. We are flying Aer Lingus which is very reasonable. You can fly into Ljubljana also. Lake Bled in that region is, according to many friends, a must visit.
Have fun.
The length of your trip is a critical bit of information you haven't provided. And do you plan to use public transportation, or will you rent a car for part or all of the trip?
Flights are probably expensive because August is still part of the peak period of travel in Croatia. You might find flights significantly cheaper if you push the trip to the first or second week of September, into shoulder season.
If you want to include Slovenia, consider flying to Venice and taking a shuttle directly from the airport to Ljubljana (DRD or GoOpti, maybe even FlixBus these days). If you can't get a good flight home out of Dubrovnik, a round trip to/from Venice might work if you can find a budget flight (not every day so you need to schedule precisely) from Dubrovnik to Venice and perhaps spend a final night in Venice before flying home. Or open jaw (multi-city) into Venice, out of London, Paris, Amsterdam, or Frankfurt if you can get great flights and also find a direct flight the day before from Dubrovnik.
Try to include Plitvice Lakes National Park on your agenda. It's a beautiful (though touristy) park and doesn't require strenuous hiking to enjoy.
