Should I rent a car in Zagreb to visit Plitvice, Ljubljana, and Lake Bled or take the train

I always look at transportation options as trade-offs. With a car, you (usually) get greater freedom, flexibility, and (the big one, for me) often more efficient use of your time. Going by public transportation may be fine (depends entirely on your itinerary/plans/tastes) but in some cases (including the places you've mentioned) you may need to allow more time (maybe a lot more) just to get around, so that may require you to limit your itinerary. But there are other factors: cars can be just an expensive headache when you're spending days in old, dense cities (tiny streets, difficult navigation, impossible parking, etc.). Renting in one country and returning in another may incur a large fee (how large? you have to check). And some folks are more/less comfortable/happy driving, especially in foreign places.

The short version, for me: if there's an efficient train or bus, I'd lean towards that. But if that's going to really slow me down a lot and/or limit where I can go, I am perfectly happy behind the wheel. It all depends on the particulars. But I generally consider my time being there is the most limited commodity I have to spend (you can usually scrounge some more money; it's harder to generate time...). So if a car lets me make a better use of that time, go to more places, enjoy them more...then vroom vroom, giddyup!

FWIW, in my experience, driving around Croatia and Slovenia is easy and stress-free, even delightful, and not demanding. Demanding is driving on impossibly narrow roads, on the wrong side of the road (eg Ireland and Scotland); Croatia is easy. But only you can decide what is better for your specific needs.

Thanks David. I don't mind driving and would rather spend a little more money if it gives me more flexibility and time enjoying the countries that I visit. My only concern about renting a car was parking in Ljubljana.

Rent the car. It’s easy to drive around Croatia and Slovenia. Get a vignnette for Slovenia. We did it on line. Just stay in a hotel in Ljubljana with parking. We stayed at the City Hotel. Easy in and out and just a very short walk into the city center.
Rick also recommends a hotel which was nearby. It also had parking and looked nice. I have to check out my guide book for the name. I’ll add it if no one else doesn’t.
If you are driving to Lake Bled, follow Rick’s directions to the parking lot, it was perfect and easy.

The car will be helpful in getting to Plitvice--where you absolutely must spend one night. There are buses from Zagreb, but I don't know how frequently they run in the afternoon, which is what you'd be looking for.

Before going further, check with AutoEurope, Gemut and Kemwel to see what the international drop charge will be if you pick up in Zagreb and drop in Slovenia. It's likely to be high, and there's both train service and bus service between the capital cities.

If your plans in Slovenia are limited to Ljubljana and Lake Bled, I don't see that a car will help you at all, really. There's bus service to Lake Bled. You can also get to the Postojna and Skocjan Caves by bus--though Skocjan may involve a bit of walking.

David sums it up very well. I did public transport between all these places. It all worked, but these rides (bus and train) ranged from uncomfortable to downright miserable. If you’re cool with renting and driving, I think it’s preferable. The roads all seemed good, not much traffic and felt like it’d be pretty smooth driving. And taking a car into Ljubljana or Zagreb is a much different proposition than driving into Paris or London.

Our drive from Plitvice to Zagreb was very easy and gave us the flexibility that we wanted. The terrific host at our B&B in Plitvice warned us of numerous traffic cameras and police with radar along the route who enforce strict speed limits. She was absolutely correct, so keep that in mind if you do decide to drive.

Based on all of your suggestions, I am going to rent a car. Thank you all very much.