Hi Manda,
Sue and I went to a large site near Porec in August 2004. Have to say I totally agree with Lutz's comments. Very glad we went, had a good time, but difficult to identify MUST SEE things. A lot of the Istrian Peninsula looks just like the rest of the Istrian Peninsula.
What made it a good holiday? Well, the welcome was very good, people very friendly - (all desperate to try out their english so my rather poor German had almost no outings - all menus in multiple languages inc english. Actually it is a little disapointing how much english is spoken on the continent these days.) The sea is really nice for swimming and snorkling. Almost no beaches, but the whole coast has loads of flat rocks which are fine for sunbathing and then you just drop off the "shelf" into the water. The highlight of the holiday was definitely the fish supper cruise which took in Limski Fjord.
Have a good time,
Mike A