tips for best places to visit driving from Venice to Split

Ljubljana is enjoyable as is Rovinj if you decide to travel through Istria. Piran is another option - especially if you want to stop in Slovenia but decide against Ljubljana.

In 3 days, I would not go all the way to Zagreb, it is quite a detour. Same for Plitvice lakes: they are gorgeous but there isn't that much to do there - I would only consider them if going between Zagreb and the coast.
Trieste is an interesting stop, you could then spend days 2-3 in Istria (Rovinj or Poreć), and reach Split on day 4.
If you need to reach Split on day 4, then skip Trieste .
Last but not least: will you drive back to Venice after? Or are you considering renting a car in Italy to drop off in Croatia? The latter is expensive and you might want to take the ferry from Venice to Pula (Istria) and rent a car there.

Plitvice Lakes national Park is fabulous, but it's really, really important to spend the night very nearby so you can get into the park when it opens the next morning. Otherwise, you're likely to find yourself part of a mob packed almost shoulder to shoulder on the walkways--which seriously degrades the experience. So the question is: Can you craft an itinerary that fits with that timing at Plitvice"

I think it's a big mistake to allow only one full day in Venice. So many people don't like Venice after a short visit like that, because they spend almost all their time in the same few locations as the day-trippers. Although I like Zagreb a lot, it isn't Venice.

I would agree with several suggestions up thread. Have you been to Venice previously? I think it warrants more time, eg. 3-4 nights to experience what Venice offers. We spent a week in Venice prior to visiting Slovenia & Croatia. We felt it complimented the history of Croatia. I can't comment on Trieste, but if we go back it would be a great stop. Ljubljana was one of our favorite stops. We spent 4 nights and didn't want to leave.

Yes, Plitvice Lakes is a "must do" if you can spend the night & hike the next morning. With limited time you may want to consider taking the ferry from Venice to Pula or Rovinj. We enjoyed Rovinj very much. We stopped in Opatija for lunch which is another lovely seaside town. Are you visiting Hvar & Korcula? We especially enjoyed our 2 nights in Korcula. Wonderful cuisine & wines.

KBK is totally right about Piran. On our trip, we departed with our rental car from the Venice airport, and stayed the first night in Piran. We made time to see more of the town the next morning, before moving onwards.

Like most posters write, if you only have 3 days, Zagreb is too far of a detour. Trieste isn’t that interesting a city if you don’t have something specific to see in mind. I’ve been there quite a few times and never plan on going back. The Miramare Castle is about it for me. Pula has a Roman colosseum which is quite impressive and has a cute pedestrian only thoroughfare. A little bit out of the way though. Any of the cities along the coast are a good bet. There is Zadar, Šibenik, Trogir. A few islands too, such as Mali Lošinj, Rab, Hvar, Korcula. Are any of these places on your boat tour? Are you driving or going by bus? All these would factor into where I stop for the day/night. Each city has their own charm and similarities. You really can’t go wrong with whichever place you chose. Plan for convenience and ease of transportation.

Are you aware it’s going to cost close to $2,500 to rent a car in Venice and drop it off in Split? When you said three days, do you mean two nights?
I would take a ferry from Venice to Rovinj (3h): https://ssl.directferries.com/ and spend a night or two. You can rent a car in Rovinj and drive to Split and drop it off at the airport (6h 45m). If you spend one night in Rovinj, add another day in Venice.

We eventually turned our car back in in Venice, so no extra return fee. We stayed in Venice at the end of our trip.