We recommend shopping around for the best connection and cheapest price based on your travel dates, prices can vary depending on the time of the year you are traveling.
If you fly from anywhere else in the States to New York, there are flight connections from NYC to Venice that are fairly priced.
If you are flying into Venice airport, all transfers from the airport stop in the city center – we recommend you take advantage of that and spend at least a few hours being a tourist. Venice city center and airport are very well connected, with frequent train, bus, and vaporetto (water bus) connections.

We would not recommend renting a car in Italy, mainly due to the reason that you can’t return it in Croatia.
Also, keep in mind, car rental is much more expensive here in the EU than in the States.

shuttle from the airport departs twice a day for Rijeka, there is also regular bus lines available every hour. The shuttle ride from Zagreb airport to Rijeka is about an hour and a half drive and the cost is $23.
Buses are more budget friendly, starting at $12, the bus ride is usually at least 30 minutes longer than the shuttle ride.

Ljubljana airport is a very well connected, however buses to Rijeka are not frequent, please ensure to check the schedule prior to arrival.

you can take the airport shuttle to the city of Trieste and then connect with a bus to Rijeka.

you can take a fast train to Trieste, and then bus to Rijeka. Train costs 13€, and the ride is about an hour and 15 minutes long.
Buses from Venice’s international Marco Polo airport to Rijeka depart twice daily.