Lax to Croatia then Italy

Zagreb is Croatia's major airport. I was able to fly home from Zagreb in 2015 with just one stop (in Europe). I assume the same would be possible for you. Check fares for multi-city tickets into Zagreb and out of Venice as well as round-trip to Venice.

Be aware that flying home from Venice may well mean an extremely early departure from that airport, too early for public transportation to be available. That would require a horribly early wake-up call and costly private transportation to the airport. Take a look at flight schedules from Venice to LAX. If they are bad, you might consider going to Venice first and returning home from Zagreb. You'll probably still have an early departure, but it shouldn't cost you $100 to get to the airport.

For Plitvice Lakes National Park it's very important to spend the night before your park visit at a hotel in or near the park. That will allow you to see much/most of the park outside the horribly crowded day-tripping hours. Effective this year, you are only guaranteed entry to the park if you pre-purchase an entry ticket. There's bus service to Plitvice from Zagreb and Split, but a rental car would make things easier.

Do you want to see any of Croatia besides the national parks? Plitvice National Park is very popular and worthwhile but while there are others in the country that's not the reason most people go to Croatia. Do you have any interest in the historic cities of Split and Dubrovnik (or Zadar, Sibenik, Trogir, etc.) or the islands (Hvar, Korcula, Brac, Vis) or the Istrian Peninsula - picturesque fishing villages and hilltowns (and roman ruins in Pula, 5th century Basilica in Porec). So without knowing where else besides Plitvice you plan to go suggesting routes is difficult.

But the main ways to get between Croatia and Venice are either the ferry (goes to Rovinj and Porec from Venice) or there is bus service that drives you between those towns (Venice and Rovinj at least). Of course you can also fly between Venice and places like Split and Dubrovnik.

I think starting with doing some research and deciding what you want to do, what you don't, and how much time you want to spend at each would be best. Nail the key factors down first. Forum participants will be able to be more helpful if we know exactly what your priorities are. For example, start with the basics. Are you limited to travel specific days of the week or dates? What's your budget? (Flying into different airports on different days means different costs.) Will you have a car? The answers will influence what you can do and when.

Personally, I think it would be a shame to go all the way to Croatia and only see the parks. They're nice, but there's so much more. Do you have some time/interest for doing more? If not, and if you only want to do two of the most famous parks, Krka and Plitvice, and get out of Dodge, I'd recommend flying into Split, as both are easily accessible from there. Zagreb is closer to Plitvice, but it's more difficult to get to Krka from Zagreb. Again, depends on your priority, time/date/transportation constraints, and priorities.

Based on my experience, people are right about Venice. It's always very early morning flights and it is expensive to get to the airport. Venice first, then Croatia/Split would be easier in that the flights out of Split to Europe, where you would need to connect to get back to LAX, are easier. But, are there flights from Venice to LAX? I don't know. Another variable. Still, Split is big enough to give you a range of airlines and prices to Europe but are you limited/interested in flying Star Alliance or another carrier group? How tight of a connection are you willing to have? All issues that would/could limit your choices. For narrowing down your options, I'd try starting with Google flights if you're doing all the planning yourself, or a good travel agent if you're not.

Remember that there are no direct flights from Croatia to the US, and especially LAX, at that time of the year. You'll have to transit through Europe somewhere.

Many sobe, airbnbs, B&Bs, hotels are of good quality in Croatia. Costs vary, of course. What's your budget? What best fits your style? Once you have a clearer idea of where, when, how and how long, you can proceed with accommodations. You've got plenty of time for that.

There's a direct seasonal American Airlines flight between Philadelphia and Dubrovnik that will run until late October in 2020. If that would work, you might look into flying to Italy (although it might be Milan or Rome rather than Venice) and back from Croatia. It's not clear which national parks you want to see in Croatia or how long your trip will actually last, making it difficult to provide accurate advice.

I realize that Italy (Venice) and Croatia look like they should be easy to do in one trip but flights aren't always available especially in the shoulder seasons like Sept/Oct from the cities like Split. I think I would pick either Croatia OR Italy.

Now depending on how long your trip will be you might be able to hit both considering you are wasting one whole travel day in airports.