Venice to Croatia

Do you plan to rent a car, or will you be using public transportation?

Thanks. we are renting a car in Venice. It looks like I will take a ferry one way and then drive back along the coast to Venice

I like Ljubljana a lot, and I'm sure Rijeka and Trieste are also worthwhile, but four days isn't very long, and in your shoes I might take advantage of the car and really see the Istrian Peninsula. I traveled by bus and was somewhat limited in where I could go. I stayed in Rovinj, which is very nice. The old town is a pedestrian zone, so investigate parking options before comitting to lodging in the historic area. Porec is another very pretty coastal town, with a very different look. Both get a lot of tourist traffic. The very small town of Vrsac, also on the coast, doesn't seem to get the crowds and is worth a quick viskt. Be sire to drive inland to see some very nice historic towns with fewer visitors. I liked Motovun and Groznjan.

My number one recommendation would be Plitvice Lakes National Park, but only if you were able to spend the night nearby and get an early start the next day (and you'd want decent weather). It gets swamped by day-trippers to the point that enjoyment of the park is seriously affected.

Susan, the ferry is not actually a ferry, but a catamaran and does not transport cars. That said, it takes longer than driving anyway - 4 hours vs 3 by car.

In your timeframe, I would spend a couple of days in Ljubljana and Bled in the Julian Alps, followed by a couple of days in Istria, enjoying a variety of scenery and staying within close range of Venice. I agree with acraven that Plitvice is well worth a visit, but I would leave it for another time, especially given your very limited time - it's out of the way and quite a long drive.