There is so many wonderful places in the center of Istria, that I will recommend to you one day trip to visit Cave Baredine and little town Grožnjan. Also there is on the way the town Motovun, little town on the top of the hill with spectacular view and wonderful sunset.
One day trip have fiew stops, so you have to plan your time in each place.
Cave Baredine have organized tour every hour fiew, and have guide on multiple languages. Official website of Baredine is HERE.
Near the cave there is a museum of tractor very interesting and fun for the children.
After Baredine, in the 20 min of driving you can have perfect romantic time in the quite little town at the top of the hill called Grožnjan. With so many little galleries, and some restaurants you will feel like you travel to some other century.
Also on the way home you can visit Motovun, perfect town on the top of the hill. Perfect view with wonderful sunset can give you unforgettable time.