We are staying two nights in Rovinj, and wondering what "must see" things we should try to do? Would definitely like to go to Pula for part of a day. We have a rental car. We'll be driving from Rovinj to Plitvice Lakes after, so if there are any significant sights on the way there, that would be good to know too.
You could visit Porec (I skipped it, but some people rave about it).
You could visit a few interior hill towns - popular among tourists.. Motovun is the biggest and most touristy. Groznjan is much smaller but has been almost completely restored and feels all fixed up - and is lovely. But it takes more time than you'd think to try to drive from Rovinj to these places and back, and with only two nights, I'd probably limit your excursions. I'd probably do Pula and not much else. Pula was a bit of a letdown for me. I've seen great Roman ruins...in Italy, so seeing more in Pula didn't excite me, and I didn't find the town all that charming (though at least it's a real working city. Tiny Rovinj, which I adored, has gotten very touristy, even though it's still a fishing village, too.)
You might try detouring to a couple of the hill towns on the way to Plitvice, if you don't mind adding a few hours to your drive. The towns aren't exactly on your way. Depends on how early you want to depart Rovinj and how late you want to get into Plitvice. FYI I enjoyed walking around the harbor of Rovinj in the morning - very peaceful, nice to see the fisherman getting ready, stray cats checking things out, before the tourists arrived. Rovinj wasn't a place I was keen to leave at the crack of dawn.
An easier stop on the way to Plitvice might be along the Kvarner Gulf - towns like Opatija might make an easy stop, not far out of your way. I wasn't crazy about Opatija as a place to spend a night but I enjoyed a quick hour tour of the place on the way south to Zadar.