To begin our Croatian/Slovenian trip, we plan first to fly to Zagreb, rent a car, drive to Ljubljana, then drive back to Plitvice Park and on to Split, leaving the car there. Question: I see that there are tolls on the road from Zagreb to Ljubljana. Will we need Kuna to pay the tolls? Euros? Credit card? I'm trying to figure out what currency we need when we arrive. Also, I understand there is a sticker necessary for driving in Slovenia. Can that be purchased at the border?
You'll need to purchase a vignette driving sticker at the border for Slovenian highways, yes. I have never needed one because I've never driven a car into Slovenia; my Slovenian rental cars always came with one included. Slovenia uses this instead of the tolls that Croatia uses.
Get some Kuna from an ATM when you arrive in Zagreb, just in case, to pay the tolls on the Croatia side. I was able to use a credit card to pay mine (US chip and signature card - no PIN required) but I would still want some cash handy, especially if you are headed back to Croatia later in the trip.
We recently (last week) rented a car in Split and drove across the Slovenian border. While on the highway in Croatia we stopped at a convenient "autostop" for gas and were able to purchase the Vignette sticker there before crossing into Slovenia. Look for the orange sign in the window of the gas station that says "vignette." Yes, have Kunes for the tolls in Croatia. We drove from Split to Plitvice and paid a total of 70-100 kunes in tolls. You won't need euros for tolls in Slovenia with the Vignette system.