I don't know Lokvarka cave, but it should be doable on the driving day from Rovinj to Plitvice.
A friend of mine did recommend Pazin cave (Pazinska jama), about 40min drive from Rovinj. It's more about adventure.
https://pazin-cave.com/en
We are in Croatia right now but did not visit Pazin cave, because we visited Skocjan cave already on our driving day from Slovenia to Pula. That was really impressive.
https://www.park-skocjanske-jame.si/en
A couple of years ago, we did drive from Plitvice lakes to Zadar and did a short detour to visit cave park Grabovaca and visited Samograd cave. It was early June and beside us (family of 4) there has been only another couple for the guided tour. Thus it was a very personal tour with a lot of time for fotos and questions. I enjoyed it a lot.
https://pp-grabovaca.hr/en/
If you have some more time the Memorial Center "Nikola Tesla" Smiljan is not far away.
https://mcnikolatesla.hr/en/