I’d like to visit the hill towns of Istria, using Rovinj as my base. Can I rent a car for a couple of days while there, instead of reserving one while in the states? I’d like to be a bit spontaneous. From what I’ve read, bus service is sketchy to these locations. I’ll be there the end of April/May.
In 2017 we drove from Slovenia to Rovinj for a few days and did a day trip to several nearby hill towns. It was easy driving. I haven't checked but would be shocked if you can't find a car to rent in Rovinj. Can't remember if I had to show my International Drivers Permit when renting the car (at the Zurich airport) - check to see if you need one.
I would google rental car in Rovinj. That may provide you with the info you need.
April May maybe you can play by the ear (not so jam packed)