drive from Trieste to Croatia?
 

Is it possible to drive from Trieste to Croatia and if so, what is the distance? I've got 2 nights planned in Trieste and was hoping we could day trip down to Croatia and see a little of Slovenia on the way back, but I can't determine driving distances - are the northern, coastal towns in Croatia worth the drive or should I save the trip when I have more time to explore the rest of the country?
Are there any issues with crossing the borders in a car (my personal car, not a rental)?

Of course it is. Distance between Trieste and Rovinj is about 100 km or 60 something miles. Depending on the traffic and waiting at the borders, this trip shouldn't take more than 2 hours. Passing borders is hassle free, but if you will be travelling in July or August avoid weekends due the heavy traffic and long waiting at the borders.

Which is better - driving down from Trieste for a long day trip and exploring Porec and Rovinj or taking a hydrofoil over from Venice for the day?

I can't really tell. I have never taken a hydrofoil from Venice or the other way around, but have many times traveled by car from Istria (northern Croatia) to Italy or Venice and it was always hassle free. Borders are very frequent used by thousands of tourists each day during summer and there is no problem at all.
But I could imagine hydrofoil day trip could be fun but in this case you'll probably have to stick to just one place (I reccommend Rovinj).
Because I have never traveled by hydrofoil on this route I would choose hydrofoil, but I can't tell which is better. I guess by hydrofoil is more relaxed, you do not have to worry about parking, driving, passing borders, traffic conditions etc.

You probably have the insurance to cross into Slovenia and Croatia so you just drive there. If you don't have the auto insurance do not go there.

If you haven't seen Ljubljana, Slovenia yet I would go there first and then see Trieste after you've seen everything else in the neighborhood.

Those countries are small so the distances are not great. But the mountain roads can slow you down so you won't make a lot of distance in an hour. No matter, unless you're in a hurry to get somewhere special.